II

迅捷突击

Can be sold eventually if a counter is needed, you will have lots of fire rate from other sources later

III

主动

异教献祭

Priority sell in the late game if/when a slot is needed

IV

幸运一击

For maximal damage in a very long game, but hard to fit when counters needed. Could sell Titanic Mag for it if you can deal with a bit less ammo.

IV

破防术

Shreds through Yamato ult, Viscous ball, Colossus, etc