"And of the Assault Marine so do I decree: He shall descend upon the perfidious foe as an Angel of Judgement from on high. Let the jump pack be his wings, and the roar of its engines a hymn of retribution. Let the chainsword be his sceptre of decree, its harsh voice singing joyfully with each and every blow. With it shall the Assault Marine bring bloody retribution to the heretic, the traitor and all alien aggressors who trespass on the Emperor's domain. So will the Assault Marine be the hunter of warlords and the slayer of kings. His armour shall run slick with the life-blood of the vanquished, and all shall honour his name."
— The teachings of Primarch Roboute Guilliman, the Apocrypha of Skaros
Where the core of Legion is the Tactical Squad, then the Knife, or better yet the Dagger of the Legion is the Assault Squad.
Rules and Tactics-wise, the Legion Assault Squad is no different than an Assault Squad from Codex: Space Marines or Codex: Blood Angels. When fielded en-masse they can really put the hurt on anyone in the brutal art of Close Combat. Twenty bodies with 3 Attacks on a charge and Hammer of Wrath can seriously ruin anyone's day.
I do have some profane love for Space Marine Squads of 20, that really, I create Tactics that revolve around sheer wait of numbers, and lately it seems, hording is the way to go, especially with Legion Armies. Like the Tactical Squad, purchasing the extra 10 Assault Marines is extremely cheap, hence it really is best to take as many as you can. But this also leads into the MAJOR drawback of the Legion Assault Squad!!
The additional Assault Marines cost 15 Points per model, and that's perfect!! But the basic cost is 250!! That's just way too expensive!! If you want a thematic army (like a Raven Guard Legion) than take the Legion Assault Squad, but personally, I cannot see this being used... Sorry my beloved brutal, tear the face off an enemy in close combat unit...
Melta Bombs - Nice, but you have to take it for the whole squad so it becomes too expensive. Would've worked well if you could take it for a few models.
Combat Shields - Again nice, but it does become rather expensive.
Power Weapon - This I like. Power Swords or maybe a Power Maul is the way to go. If you're picking targets, you should avoid most things you can't kill.
Hand Flamer - Fun, but I'd much prefer the Power Weapon, as you'll most likely be too far away to use it or in close combat.
Plasma Pistol - Too expensive!!
Hand Flamer - Like I said before, fun, but you might not see it used as often as you'd like.
Plasma Pistol - Too expensive!!
Power Weapon - Power Sword or Power Maul, your choice. Power Axe would be nice, but I'd rather take the Power Fist.
Single Lightning Claw - I've always believed Lightning Claws should be taken as a Pair or with a Storm Shield perhaps (unless you're in Terminator Armour). So personally, I say no to this option.
Thunder Hammer - I love Thunder Hammers, but a Power Fist would be better for you here.
Power Fist - If you take this, you don't need Melta Bombs. Coupled with Artificer Armour, you can prove to be quite deadly in Challenges.
Pair of Lightning Claws - Well yes it's better to take these in a pair, but I do have to say they feel a little bit too expensive.
Melta Bombs - Since it's better to take Melta Bombs for a few models, the Sergeant will suffice.
Combat Shield - Originally I was gonna say no, but it might possibly be worth it. It makes the Sergeant a tiny bit more survivable in a Challenge.
Artificer Armour - With the new Challengers, this might actually prove to be a really good choice.
Suggested Option 1: 468 Points
20 Legion Assault Marines with 4 Power Weapons (Power Sword would be preferred). Sergeant has Artificer Armour, a Power Sword, Combat Shield and Melta Bombs.
Like I said before, too expensive. If it was around 50-90 Points cheaper then I'd see these guys competing with the Legion Tactical Squad for Primary core unit. But make do with what you got ay? Sheer wait of numbers and drown your opponent in numbers. Utilise the speed of the Jump Pack and try and assault the opponent first. If you use Rites of War: Angel's Wrath, you could really give the pain!