Charging a weapon is considered a FULL attack and cancels away sprite's cloak. Shield does not. Shield + attack (simply starts charging, no swing is made) simply cancels cloak...
Add the fact you can't shoot while charging and there you have it: battle sprite can only be used outside of battle.
My suggestion is either:
a. remove battle sprites, their effect on the game is negligible
b. give them a meaning! allow usage during charging of weapons, allow charging weapons when cloaked (invisible).
There is no point in sneak attack if it does no extra damage. Non-charged attacks do very little damage to stronger monsters, and don't have much of an effect on bosses.
Remove universal cooldown and it will be like this:
apply cloak
charge blitz needle
use poison ability
shoot blitz needle with small damage bonus and poison damage bonus
run away for 40 seconds
repeat
...
profit!
How it looks now:
shiver to freeze vana in place
charge blitz
shoot blitz (unable to apply poison - doesn't last long enough to apply poison before charging blitz while dancing around big angry guards and spamming shiver)
Current effect on the boss fight: unnoticeable.
Why do I have a battle sprite again?
"Why do I have a battle sprite again?"
Because people begged and pleaded for them.