Infiltrator:
In this order of getting them:
Assasin cloak
the 100% paragon/renegade option untill you max out your bar, then respec it to the other more damage one
Heavy incinerate
Heavy disruptor ammo
Heavy warp ammo
lvl 1 ai hacking
the last 3 should be intermingled a bit in their order of getting them (getting lvl 3 in each before getting lvl 4, etc)
Guns:
I got kasumi's DLC when I was half done the other loyalties, if I were to go back I would do hers near the beginning to get the SMG from it
Viper sniper rifle, swapped to widowmaker for the thresher maw and the last boss
Locust SMG
Predator heavy pistol
I only used my heavy weapon on 3 bosses, and it was the cain on the thresher maw, the reaper core and the reaper larva.
Armour:
Terminus armour until you do the collector ship mission to unlock these armour pieces:
Kuwashii visor - +10% headshot damage (alternatively recon hood also is good, but additionally looks much less cool)
capacitor chestpiece +10% faster shield regen less time wasted between assasin cloaks to wait for recharge
N7 shoulderguards +3% weapon damage
Stabilization guantlets +5% weapon damage
Stimulator conduits +10% storm speed (move further while stealthed)
The key here is maxing your weapon damage, as every % bonus you get stacks amazingly well
All of these are bought in Omega or are your basic gear, so drop by there after you finish the collector ship to get them all (they are only available then)
Here is some rediculous math to show how much all of these bonus's stack up.
+ Show Spoiler +
Total bonus damage on widowmaker to the head of an armoured target: (this is just to show all the different sources of increased damage possible)
+18% gear
+15% Operative skill
+50% base weapon bonus of widowmaker vs armoured
+50% base weapon upgrades
+unknown% for base headshot bonus, possibly 100%
+50% headshot upgrade
+50% anti armour upgrade
+50% warp ammo
+75% assasin cloak
I don't know which or these are simply additive or which are multiplicative I think it is somewhere in the middle (for example, the anti armour upgrade and the base anti armour property are just added to make it 100% vs armour, but the assasin cloak is added on at the very end as an additional +75%
Allies:
Garrus, oh god garrus
such an absolute powerhouse of a character to have with you at any point in the game.
reasoning:
Overload vs shields/synthetics
impact shot vs biotics/biological
Heavy armour piercing ammo, for armour/biological
As you only get 12 pts to spend across his 3 abilities it's a toss-up whether to spend any on his 'special training' ability or not, as it prevents you from getting the above 3 as heavies
always using the incisor sniper rifle
He is the one to take with you when you have to carry another nearly useless character for their loyalty mission.
Zaeed (use until you get thane)
Nothing special, get impact shot and his training, as he gets the same +50% damage thane gets
Using the incisor sniper rifle of course
Thane
his +50% damage from his training plus his heavy warp are just too juicy to pass up
max his training and warp, then throw the rest into shredder ammo, not the best ammo type, but since we're not using squad ammo at all, it's bonus damage
and as always, the incisor sniper rifle for the weapon.
general strat:
max out your upgrades for the sniper first, then the SMG, then the pistol.
always swap between your weapons so that you don't have one out of ammo and the others at full. Each ammo pickup fills each gun by their set amount if they are not full. This means being 80% on all 3 guns = 1 ammo pickup fills you up, instead of being 30% on one gun and needing 3 ammo pickup to fill up.
Use your ammo types and assasin cloak. 1 clip from a fully upged locust at lvl 30 from assasin cloak with the appropriate ammo for the enemy will almost exactly down any trooper level enemy(I was finding I had 2 shots left on the 20 shot clip vs collectors in the suicide mission. This is incredibly usefull when a trooper decides to charge down the side path at your cover
Husks die in 2 heavy shots, ie heavy warp from thane + heavy incinerate from you. just back up continually spamming these abilities and shooting with SMG or pistol. I rarely find it worth the ammo to use the viper on them. Just remember to always tell your allies to run away if a scion appears. I generally take these out from outside their range with assasin cloaked viper clips, THEY are worth the ammo!
For harbingers:
warp/impact from allies straight off, followed by an assasin cloaked full clip from the locust with warp ammo should get him into his armour, then incinerate + a few shots from garrus with his armour pierce ammo should finish it off. If not another warp from thane will help as well if it gets to that.
ymir 1 or 2
hide your allies somewhere
these are actually rediculously easy to kill if you're an infitrator due to this tactic:
if you know you're going to be fighting a ymir set your SMG to disruptor ammo and pistol to warp ammo
assasin cloak, full round of SMG into it's shields
repeat the above with a small delay between the CD of assasin cloak and when you go again, as you will take a few hits of damage you want to regen each time
This will cause the ymir to spin up it's gun, begin firing right as you go into cover, lose you due to assasin cloak before it starts it's rocket, stutter step in place, moving towards you slightly. you pop back up, shoot it again, and repeat. just do this until it's dead.
the key is to regen shields between each hop up as well as reloading to ensure a full clip of either SMG for it's shields/health or pistol fire for it's armour
boss tactics:
I catually don't know if it makes a difference, but assasin cloak before firing the cain for extra damage, this will do about 25% of the thresher maw or reaper larva's hp in 1 shot and 1-shot the reaper core (making you only have to endure 1 opening of it's shields)
the praetorians are really easy, just place your allies far enough away they won't trigger his slam, but close enough they can shoot, and kite him in circles around a LoS blocker. He will never go after your allies as they peg him down. take a few pot shots as you want. it will take a little while, but it is hard to die vs them when doing this. For the one in the collector ship, since it comes with husks and collectors, hardcore focus those down first (assasin cloak + opening up with viper)
it will slam it's 3 times (75 50 and 25% armour) and regen it's barrier each time. the only way to prevent this is to cain it.
On the reaper larva or thresher maw, assasin cloak + heavy warp ammo'd fully upgraded widowmaker shot every time assasin cloak is off CD will tear both of these down so fast you won't even have a challenge. if you are running low on ammo both these fights have infinite ammo (reaper larva through the fact that every harbinger you kill drops a heavy weapon ammo box) on my second playthrough I only had 1 harbinger I had to worry about the entire last phase of the reaper larva fight it was over so quickly.
If you are having trouble with an area as an infiltrator, run through it in stealth, if you hit an auto-save checkpoint and then die, even with no enemies behind you dead, when you repop they'll all be gone and you can go back to loot any safes/tech they were guarding. This got me through my first insanity playthrough, though I tried not to use it on my second.
the place I still used it was in tali's loyalty mission, I just booked it through to her dead father, which cutscenes, despawns everything behind you, and revives your allies with you.