I use the assassin, and generally do very well. I play her a lot more defensively than most people seem to, I always use her blade gun to rack up a lot of bot kills/assists, upgrade turrets after I get her sword and cloak boosted, and use the sword at people trying to get into our base/jackbots. I rarely end up dying much and rack up assists and kills this way, then if a jackbot comes I can just rush it no problem(I use the gold armor for her). If it goes into overtime, assassin's are crazy good at just cloaking over there and lashing out on the ball.
Some stuff people don't seem to know that helps a LOT are using her flashes to disable turrets. Support with an overhealed sentry right outside of your turret areas? Shoot at it to annoy him, cloak and flash it to disable. Then either melee him or sprint around slashing his sentry. Getting rid of either one works!