By the way, how do I know when and how many times I can use the X-boost?
The circle around you indicates the status of your boost-mode. Empty, it's a tiny red little thing that circles around you. It charges as you're not holding the attack button down, and as it does it gets longer and turn white, then blue. When fully charged it turns into a solid circle. Obviously, it discharges as you use it, but I don't know the specifics of how much, since I rarely use it. (Summoners don't really get the chance to lay off the attack button very much.)
The summoner's bomb is sort of like an un-summoning circle. It cancels all bullets within the area of activation, yeah, but also, any enemy close enough to the center will start to turn sideways; once they're fully turned, they take damage - enough damage to instagib most enemies, and even makes pretty short work of heavies. Unfortunately, it doesn't really last long enough to work on more than one grouping of enemies per activation. I've confirmed it
does work on bosses, but the numbers looked to be about half my basic attack (though I don't know how the DPS stacks up).
Also, wtf @ Annex Palace bosses?