The way damage on that boss works is that once you deal enough damage to destroy a clump of shards, it will automatically deduct 4 from the boss' total hit points regardless of whether or not you overkill it. There's no way to make the fight faster than killing each of the 7 clumps the frame they can take damage.
Zwei's maximum speed is equal Tia's sliding speed(6px/f) and takes decent amount of time to accelerate to that velocity, therefor it is only useful in rooms the where terrain forces a lot of jumping(airborne horizontal movement speed is 3px/f).
Also, I am refilling Zwei constantly throughout the fortress stages. The
Zwei ammo refill route is very specific since you need just enough ammo for each Zwei to expire when you jump off of it so you're not wasting frames filling up excess, and there are only a limited number of enemies between each Zwei to actually pull this off. For instance, using Zwei in the long checkpoint room of Iris 1 is 1 frame faster than using multiple Eins, but there simply aren't enough enemies after the preceding Zwei in the 3rd room of the stage without resorting to picking up a big ammo drop(which would cause 10 frames of refill lag, completely negating the time save, and you wouldn't be able to get the 4 ammo you need for the Zwei in the last yoku block room of the stage since there is only one enemy in your path before it).
