Tetras, in a way, break the game. Not because they're too good, but because they're so fast. They accelerate the pace of a match tenfold and move from target to target faster than any other weapon. Your job as a Dark Tetra is to just annoy the enemy. Dodge in, drop a guy to low health, dodge out. You don't need to get excessive amounts of kills to do amazingly with Tetra. Simply be annoying and let your dedicated slayers (Tentatek Splattershot, Enperry Dualies, Blasters, etc.) finish them off.
Since you're constantly harassing the enemy, you're probably going to die more often than most. Quick Respawn, Stealth Jump, Quick Jump, Ninja Squid, and Swim Speed allow you to get back into position quickly and safely. Tetra has Splashdown, so don't even attempt to build it most of the time, as it won't be very useful.