If you're playing Tyranid you need to understand the (overly) complicated close combat rules in 4th edition 40k if you want your troops to be as effective as possible.

With genestealers we have probably the best close combat unit in the game - their cheapness, rending, the fact they're a troops choice and their initiative of 6 makes them kings amongst assaulters.

Even if something like Harlequins is that little bit nastier, remember, the Eldar can only have 3 units of them, and your stealers are almost as good.

However, they have one main weakness - they're extremely fragile.

The keys to using them succesfully in combat are to prevent your opponent striking back, and to try and ensure you're safely in combat on your opponents turn. This will let you attack, kill and then survive at sufficient strength to do it again and again to other units. Thanks to rending and I6 this is surprisingly easy. Once you can reliably assault, kill, and not lose many genestealers in the process, they change from being a unit of mutually assured destruction into a unit that can wipe out large sections of your opponents army.

1) Fun with initiative

"A model will only fight when its initiative rank is called." Pg 39.

As a model can only attack when its initiative rank is reached in CC, you can deny a model any attacks by ensuring it's not engaged by that stage. A model is only engaged if it's either in base-to-base or is within 2" (3" for beasts) of another model from its unit that is in base-to-base.

Cool things this can let you do:

Stop that powerfist swinging:

Powerfists always strike at I1. So if you can remove the base-to-base models within 2" of the powerfist at higher initiative level (because they were casualties) then the powerfist doesn't get to strike. This is one of the, Initiative 2, CC carnifexs few defences against powerfists, the other is killing them first :) It doesn't matter how many armour ignoring, high strength attacks that powerfist could make IF they're not engaged when initiative 1 comes and hence can't strike.

Stop an entire units attacks if it's higher initiative leader goes first (with luck).

Some units have a higher initiative leader, eg a broodlord with stealer retinue or a I4 wolf pack with I5 leader - this can be a disadvantage for the unit.

When the higher initiative guy strikes, you can chose to remove the models within his killzone that will dis-engage the largest number of the rest of his unit. So, if for example, he has 6 guys within his killzone, and 3 are basing the rest of his unit and 1 is basing him. If he kills 4, and you remove the 4 models basing, then the rest of his unit gets 0 attacks! If instead you leave models in base, then his unit gets to attack! You might want to do this if you need some left in base for your own guys to strike back. However it may be worth sacrificing their attacks to receive less yourself.

You also may choose to leave some in base so as to be able to sweeping advance, as you cannot without any models in base-to-base (see page 43). If you're necron probably the last thing you want is to give your opponent the chance to sweeping advance though! See how, even though you're simply removing casualties, you can have a major effect on the out-come of the combat. So think casualty removal through before doing it :)

Redeuce a models attacks:

Some models have different initiative attacks. For example a Hive Tyrant (Init 5) with bioplasma (init 10). If a CC flyrant charges you, you're looking at 6 monstrous creature attacks! However if he has bioplasma and you're lucky enough that he kills someone with it (in this case it's an optional attack), then you can just remove the model in base with the tyrant and deny him his 6 armour ignoring attacks. A charging tyrant will only ever base one model by the way, as they have to take the shortest route to the nearest model...

Make assaulting you a losing proposition to space marines:

Assault marines are mainly Initiative 4. If they have furious charge, or for some of the characters, they will be I5. They will tend to have frag grenades. So remember, in this case, cover is NOT your friend.

By having your stealers out of cover, you will strike at I6 if assaulted, and they will strike at I4 or I5. If the marines are still alive, that is. An 8 man squad of assault marines charging 8 vanilla genestealers in the open will average 4 dead marines on the charge. Those are dead before they even get to strike. Some of the surviving marines may no longer be engaged after casualty removal, or because they had to assault near their maximum 6", and will also not get to strike.

You are unlikely to lose the combat. Even if you do, you won't be outnumbered by 2:1, and you'll finish them off on their turn. Remember, those assault marines cost a lot more than your stealers, and you can have a lot more stealers than he can have assault marines.

If you get the assault, and the marines aren't in cover, then barring bad luck with the dice, you're going to be taking few, if any, casualties.

2) Fun with Independant Characters

An IC can only attack a unit if he is basing one of that unit. Unlike an upgrade character like a Veteran Sargeant he doesn't count as part of the unit while in combat (official 40k rulebook faq, and (not very good) explanation in main book).

So, if you can assault a unit while avoiding basing a IC, then he gets no attacks!

You have to set this up in the movement phase as in the assault phase you'd be doing well to avoid him unless you've planned ahead. With a fast moving assaulting squad like stealers or harlequins you often are able to position yourself on a side where you won't base the IC while assaulting. The IC is still subject to the effects of losing the combat though (official faq) so if you charge a unit of necron warriors (lame in combat) and avoid basing the lord (ok in combat), then you have a much better chance of winning the combat, the lord never even gets to strike, but he can still be swept!

If the IC is a different initiative because of a powerfist for example then the initiative dis-engaging approach above works even better on them. As you only need to remove the model(s) in base with the IC, unlike an upgrade character where you have to remove all based within 2". So it may just take the removal of one casualty to prevent the IC striking, but 3 or 4 to prevent an upgrade character such as a vet sargeant striking.

3) Keep them locked:

You often want a unit you're attacking to survive, so as to lock it, and you in CC for the opponents next turn. This will prevent anything shooting your unit on their turn, and may provide a useful line-of-sight blocking combat. By minimising the number of your and their models engaged you'll minimise casualties (on both sides) in the first turn of a CC. You also may, depending on the angle you approach at make it easier for you to clear your killzone, stoping them striking back. Eg hitting the end of a unit in a line means you're more likely to clear your killzone as compared to hitting the centre of the line.

In the assault phase you HAVE to use your full 6" or 12" movement to try and base a model. So once in the assault phase it's hard (see point 4) to minimise the number of bases you engage.

However you can control things in the movement phase instead. If you have stealers with a 6 inch charge, then put one well within 6" so you avoid misjudging things. Then another one 2" back, and the rest >= 2" back from that one.

This way when you charge, only one model can reach base. And only one other model of your side can make it within 2". So the most you can lose is 2 models. If you charge a good section of the enemy squad(as set up in your movement phase) you still have the attacks of 2 models to attempt to draw combat or to clear your killzone. Piles in will, against many troops, give you a good chance of ending the combat on your opponents turn. Meaning he doesn't get to shoot, or shoot you, and you're free for your next turn, with an extra 3" or D6" consolidate first :)

4) Assault rules:

As the first step of an assault, you must charge the nearest model to the nearest model in their unit.

"After the first model has been moved, you may move the others in any sequence you desire." Page 37.

With the model you chose to move next, you have to base an enemy model if possible, otherwise you just have to maintain coherency.

So if you want as few models engaged as possible (to keep the unit locked next turn) then after you charge the nearest you should charge the furthest away models that can acheive coherency with the first charger. These models will then block your nearer models and mean that models nearer, that if they had moved first would have been able to base, will not be able to. So you can end up with a lot less models based. Remember, your models may not move through other members of their unit, so by choosing the order you assault models you can, by creating a wall of models in the way, affect the number of models that end up in base-to-base contact.

This isn't a substitute for setting things up correctly in the movement phase, but it can make the difference of a model or two in base-to-base.

5) Coherency:

In close combat "Casualties must be removed in such a way that the unit maintains coherency wherever possible". Page 41.

You can't remove casualties that break your units coherency unless there are no other casualties you can remove...

This makes it easier to snipe powerfists and other special weapons.

If there's a powerfist on the edge of a group and you charge him. He cannot remove other models in your killzone before the powerfist if that would leave the powerfist out of coherency. So by basing the power-fist and making him the centre of your killzone, you can force your opponent to take him as a casualty one wound earlier than if you based a neighbouring model.

This can also affect powerfists hidden inside a squad. Depending on the squads layout the coherency rule may mean the powerfist is 3rd to be removed instead of 4th, or whatever. So more chance of killing the powerfist.

It also means that your opponent may be forced to remove the models in base contact even if that will stop him striking back.

For example: a killzone contains 5 models and you kill 3. He'd like to leave the 2 in base with you, but if he removes the 3 behind then the 2 in base are no longer in coherency with the rest of the unit. So he's forced to remove one or both of the basers. That means less of his squad will be within 2" of a based model come their initiative turn, so less of them can attack.

6): Torrent of fire

This also applies to CC, if a unit takes as many wounds as it has engaged models* at a SINGLE init level, then you can nominate a model to take a save. This happens regularly with stealers. Any other top rate assault squad such as a multi-ravener unit, harlequins, or even hormagaunts against GEQ will often trigger this rule. This is a good way to get rid of nasty things such as hidden powerfists a little bit sooner. Even a power-armourer marine has a 1 in 3 chance of failing a torrent of fire against a mere hormagaunt.

7): Mixed armour saves

The mixed armour saves rules on page 76 of the BGB seem to cause a lot of confusion to people.

The key thing to remember is that "for each 'set' of wounds, every model must be allocated a wound before any of them can be allocated a second woundr". Where they say 'set' they mean wounds caused at a particular initiative level, or by 1 units shooting.

For example:

A broadside (2+ armour save), has 2 shield drones (2+ armour, 4+ invulnerable).

If you charge with genestealers and cause 3 wounds against this unit, the first two wounds must be applied against the 4+ invulnerable save, as it is the majority save. However the 3rd MUST be applied against the broadside, as all the other models have already received one wound, saved or not. If these are rending wounds then the broadside has just lost a wound. Otherwise it attempts its normal 2+ save.

Often people, probably purely by mistake, will try and take all the wounds on their 4+ invulnerable saves.

Your opponent can chose which wounds to apply to which model , so if you have normal and rending wounds mixed together, then a sensible opponent will while distributing wounds, apply rends to his drones and normal wounds to his broadside.

However if he runs out of normal wounds to assign to the broadside, he will have to assign it un-savable rending wounds...

*Thanks for the clarification that it's only engaged, not total, models Coredump.