Ok, so I've been constantly seeing posts asking for this level, so I'm gonna provide my strategy that I used in beta and after release.
Weapon Setup
Pistol: Punisher (no mods)
Automatic: AAR
Close Range: Blunderbuss (could put a +damage mod - accuracy on it I s'ose)
Sniper: Pulsator + dual shot (doesn't matter though)
Explosive: Eggplant (shoot at feet for MASSIVE DAMAGE)
Equipment -
Offensive: Ammo Generator (more ammo for blunderbuss/aar, its a small map)
Defensive: Nanobots (so you don't die)
Utility: Don't matter
Ability: I used Katana for damage, but you could also use time shift to more easily slip in and kill the t-14
Strategy.
The most important thing about beating the t-14 and getting three stars, you MUST NOT kill him before he activates, the credit reward pre-activation is 5, versus ~25-30 for post-activation. Secondly, you want to get all 5 kills, and is preferable that you not die so you get that small kill streak bonus.
When fighting the T-14, this is the one time that the Blunderbuss is actually better than the Burster, because you can angle your shots partly past him, and they will curve into the back of his head. You can't count on him being distracted, and assassin is a lot more valuable than a normal headshot. If your Blunder runs out of ammo, you can switch to the aar and just shoot him in the head (who knows, you might still get assassin because the regions are a bit wonky), or you could shoot right below his feet with the eggplant, splish-splash (which you get from burrowing saws unless they patched it), gives 2 more credits than a headshot.
Now, as for you allies, it may be beneficial for you to let them suicide into the t-14 and save up for spree ender (25 credits), however, this may also result in a) the t-14 dying, or b) the t-14 getting a powerful weapon that is hard to compete against (the blunderbuss has extremely short range).
As a final measure, you could consider restarting so that his side-arm is not a punisher, hand cannon, early retirement, or golden gun, as they deal a large amount of damage very quickly, which will be hard to dodge (because you are so close) and hard to heal between respawns. (again, I'm not sure if they ever did fix the problem of bots spawning with premium, because I'm pretty sure I saw them after the patch)
Good luck, and have fun.
p.s. some miscellaneous tips
Credit Farming: CTF on the smallest map, 6 enemies, 5 allies (they hardly ever steal the flag). Use something with a great capability to slaughter enemies, grab the flag and start blasting away. Good candidates are burster on sg only and eggplant (burrow saws deal crazy damage) on explosive only, or just switching between burster, aar, and eggplant.
Pyromaniac achievement: Use the burster, shotguns only (or just unlimited ammo mod), ctf on the transport craft, leave pellets all over the area by their flag.
Best weapons and equipment loadout (most cases and imo)
Punisher: Accurate, damaging, kills in only a few hits
AAR: each bullet takes about a third of health, 2 bullets per burst, huge accuracy
Burster: Ridiculously long range, pellets stay on the ground, flame, close range blasts one shot anyone.
Pulsator: With the double shot mod, it comes close to almost one shotting anyone (headshots do the trick), however, snipers are rather useless in this game (I guess they learned from SFH2)
Eggplant: All of the explosives are pretty unimpressive, but the eggplant at least has the benefits of burrow, which unless they patched it (notes say nothing about it), deals an insane amount of splash in a small area.
Equipment
Ammo Generator: Powerups are much rarer, and weapons are generally greater than abilities.
Nanomachines: Although Life Support was superior in Raze 2, the buff to nanomachines (healing all your health instead of up to 40%), and the removal of health packs practically makes this a requirement.
Marathon: Being faster is always good, and honestly high grip doesn't have many uses, the backup boost is way too small, and power slide requires crouching and has a cooldown.
Time Shift: Move faster, shoot faster, and has a cooldown of less than a second.
That's all I have for now, if you have any questions be sure to ask
~~~Darth Caedus