Well, for AoE focus you'll definitely want Suppression & Venom Grenade. Optionally you'll also want Frag Grenade (if you don't mind the knockback) and/or Heavy Burst (if you don't mind drawing the rifle). You'll most likely also want the patron AoE.
Slot them as you would any AoE really - I'd say 1-2 acc (partly dependent on whether you take Combat Training: Offensive or Tactical Training: Leadership), 3 dam, then the last couple slots with recharge or endurance reduction, depending how your endurance is holding up while you play. Probably want to focus on the recharge slightly more with Venom Grenade (-res is a force multiplier, so even ignoring the damage the power does you want that -res covering everything at all times) and maybe the Patron one too (slow base recharge).
You'll also want at least one single target attack - Channelgun cycles faster and is almost an attack chain on its own (if you enhance recharge), while Longfang provides heavier hitting. Melee attacks are optional - Crabs are stronger at range (more ranged defense than other positions), but the melee attacks can provide a bit of extra single target damage if you don't mind dancing in and out of range of the nastier melee opponents. I'm not impressed by Frenzy though, even if it is a bit more (PB)AoE.
Defensively, make sure you pick up (ideally as soon as possible): Combat Training: Defensive, Tactical Training: Maneuvers (not to be confused with the inferior power pool Leadership: Maneuvers, though you can pick that up too if you want) and Fortification. Slot 2-3 defense IOs in CT
(if you slot generic level 50 IOs, a pair of 50s gives almost the same as three of them), give TT:M 3 defensive and optionally 1 endurance redux, and give fortification 3 resist and optionally 1 endurance redux.
Pick Crab Spider Armour Upgrade and, imo, ignore Wolf Spider Armour - you have to take one of them and Crab gives marginally more mez protection. Both of them only give very very minor resistances and the mez protect they give is dwarfed by Fortification, so I wouldn't burn two power choices to get both passives. I'd not bother slotting if you're not going for set IOs (it's a good place for the steadfast protection -kb and +3% def IO otherwise).
Optionally pick up Serum and slot it with 2-3 heal and 2-3 recharge - it provides a nice extra layer of protection for when things get tough.
You'll almost certainly want Fitness for Stamina, slotted with up to 3 end mod dependant on how badly you find yourself burning endurance.
If you solo I'd heartily recommend the two pet choices - Spiderlings to provide a general dps boost, and Call Reinforcements to provide assistance during tough fights; 1 acc, 3 dam, 2 rech. For teams they're not so vital, especially if you aren't relying on set bonuses/slotting to boost their recharge.
Get Mental Training too - don't turn down extra +recharge, especially if you're not getting any from set bonuses - just don't bother slotting it beyond the default 1 runspeed.
Everything else - the other Tactical Training powers, attacks, pool powers, etc, are down to personal preference and what direction you want to take the build beyond aoe. Hasten is useful with all the high recharge powers, Tough and Weave provide that bit extra protection for the aggro you'll inevitably draw with all your AoE, TT:Assault stacked with Leadership:Assault means a 30% damage boost for you and your team at all times, etc etc.
Quick example build (it does have one IO - the -kb from Steadfast - because it's not too expensive (I've bought it on numerous alts that received no funding from my other alts or other players) and it helps a lot):
Villain Plan by Mids' Villain Designer 1.401
http://www.cohplanner.com/
[u]Click this DataLink to open the build![u]
Level 50 Natural Arachnos Soldier
Primary Power Set: Crab Spider Soldier
Secondary Power Set: Crab Spider Training
Power Pool: Fitness
Power Pool: Leaping
Ancillary Pool: Leviathan Mastery
Villain Profile:
Level 1: Channelgun -- Acc-I(A), Dmg-I(3), Dmg-I(5), Dmg-I(7), RechRdx-I(40), EndRdx-I(48)
Level 1: Crab Spider Armor Upgrade -- S'fstPrt-ResKB(A)
Level 2: Longfang -- Acc-I(A), Dmg-I(3), Dmg-I(5), Dmg-I(7), RechRdx-I(37), EndRdx-I(48)
Level 4: Combat Training: Defensive -- DefBuff-I(A), DefBuff-I(17)
Level 6: Hurdle -- Jump-I(A)
Level 8: Suppression -- Acc-I(A), Dmg-I(9), Dmg-I(9), Dmg-I(11), RechRdx-I(25), EndRdx-I(43)
Level 10: Tactical Training: Maneuvers -- EndRdx-I(A), DefBuff-I(11), DefBuff-I(15), DefBuff-I(17)
Level 12: Venom Grenade -- Acc-I(A), Acc-I(13), Dmg-I(13), Dmg-I(15), RechRdx-I(23), RechRdx-I(43)
Level 14: Combat Jumping -- DefBuff-I(A)
Level 16: Health -- Heal-I(A)
Level 18: Frag Grenade -- Acc-I(A), Dmg-I(19), Dmg-I(19), Dmg-I(21), RechRdx-I(25), EndRdx-I(46)
Level 20: Stamina -- EndMod-I(A), EndMod-I(21), EndMod-I(23)
Level 22: Mental Training -- Run-I(A)
Level 24: Super Jump -- Jump-I(A)
Level 26: Fortification -- EndRdx-I(A), ResDam-I(27), ResDam-I(27), ResDam-I(29)
Level 28: Serum -- Heal-I(A), Heal-I(29), Heal-I(31), RechRdx-I(31), RechRdx-I(31), RechRdx-I(50)
Level 30: Tactical Training: Leadership -- EndRdx-I(A), ToHit-I(34), ToHit-I(34), ToHit-I(34)
Level 32: Omega Maneuver -- Acc-I(A), Dmg-I(33), Dmg-I(33), RechRdx-I(33)
Level 35: Summon Spiderlings -- Acc-I(A), Dmg-I(36), Dmg-I(36), Dmg-I(36), RechRdx-I(37), RechRdx-I(37)
Level 38: Call Reinforcements -- Acc-I(A), Dmg-I(39), Dmg-I(39), Dmg-I(39), RechRdx-I(40), RechRdx-I(40)
Level 41: School of Sharks -- Acc-I(A), Dmg-I(42), Dmg-I(42), Dmg-I(42), Immob-I(43), EndRdx-I(48)
Level 44: Bile Spray -- Acc-I(A), Dmg-I(45), Dmg-I(45), Dmg-I(45), RechRdx-I(46), RechRdx-I(46)
Level 47: Tactical Training: Assault -- EndRdx-I(A)
Level 49: Aim -- RechRdx-I(A), RechRdx-I(50), ToHit-I(50)
------------
Level 1: Brawl -- Empty(A)
Level 1: Sprint -- Empty(A)
Level 2: Rest -- Empty(A)
Level 1: Conditioning
------------
thx alot... this is exactly what i needed..
Stay away from the shampoo and tiny comb... that should help. 
So Biowraith has great advice. I made my crab a blaster/tanker/MM and I love it! I had a huntsman before so switching to this was easy. My attack chain is WAWG-Venom Gernade-Aim-Suppression- Frag. Then follow up with single target stuff. WAWG can be replaced with heavy burst if you want more aoe stuff.
I took wolf and crab armor upgrades. Mainly for the reason that if I die and use a wakie, I wont be stunned. It has great mag protection.
The most popular paths for crabs in the higher levels are taking the fitness pool, a patron for a extra AOE and pet, or more leadership stuff.
Alson you could try and pick up Tough. With Tough, Fortification, and the Armour Upgrade, you could be looking at really high res to mostly everything, while still have decent Defense to all.
My next project his gonna be a Crab with that, softcapped to all positions, perma(or close to it) pets, and with perma Serum. And that is melee mostly.
Sounds like fun.
IO'd out your crab will be a machine.
My crab is at the soft cap, 40% resist to energy/neg/fire/cld, 50% vs. smash/lethal. I've got a little over 90% global recharge (with Mental Training). This lets me keep Spiderlings out perma and Reinforcements are out for 4 minutes, down for 1.
I was more interested in sinlge target DPS for going after AV's so my attack chain has alot of single target attacks.
Currently the attack chain is Arm Lash-Slice-Channel Gun-Slice. I've got Achilles Heel Chance for Dmg resist Debuff slotted in Slice and Channel Gun.
One interesting thing I've discovered is that with the Alt build you can go Wolf Spider. You keep your crab legs, but you get the mace. Kind of crazy.
Hope this helps! Good hunting and Good crabbing!
InidiousX - lvl 50 Ill/Rad Controller
Black-Talon - lvl 50 DM/SR Scrapper
Night-Blade - lvl 50 Katana/Dark Screapper
Bone-Shredder - lvl 50 SOA
Red-Phantom - lvl 50 Elec/SR Brute
All on Champion!
Okey so im getting ready to start my first crab.. i want a aoe-focused build. Been readiing alot on the forums but i can't seem to put a build together on my own. all the builds on the forums are all Set-specced, and i can't afford to buy set for my toons. What should i focus on and how should i slot it??
THx