Another couple ways to make hordes tolerable to deal with:
1. Some sort of shockwave: What I'm thinking is an AoE wave that staggers enemies, or, in the case of Jetpack Bots, knocks them over. Either that, or it just pushes them backwards, giving you a bit of much needed wiggle room.
2. Being able to stop/slow time: Exactly what it says on the can. Being able to more easily observe the situation (by not having to worry about death) and having more time to react would go a long way towards making the Titanium rounds survivable.
I think more defensive/utility upgrades would add a lot of depth to this game, especially if they were comparatively balanced (thus adding to replayability [not that it's hurting in that department]).