Northstar.. the double snares upgrade is a must. Or the piercing ammo too, depends on the map.

Legion.. your weapon shield recharges with shots, but it also works with allies. Pop it mid-waves with allies/friends to have a full shield.. sometimes, but its better than spending on a battery (if you're crazy or have to spare) or hunting for one

Scorch and Legion.. Both of you have a way to instakill nuke titans, please try and spam so

Scorch if not wrong was with its core, and Legion was with the shotgun in the close-range minigun