Ryshin, you're correct in that, IF it is used during the Battle Step. But keep in mind the Battle Step is "breif", if you will. The Battle Step has only two "default" opportunities for chaining. I know many will disagree with me on this, but here's how it goes:
Beginning of Battle Step - TP gets priority (Assume both players pass)
OP (Opponent) declares an attack on Fusilier. - TP gets priority.
If TP passes, OP gets priority. If OP passes, we move on to Damage Step.
If TP does not pass, this would be where to activate Skill Drain. Note that if you do this, you forfeit your present opportunity to play any other card in response to the attack. Priority passes to the opponent.
If OP passes, TP regains priority and can now activate any other card in response to the attack. Same thing if OP activates a Spell Speed two effect in chain. *BUT*, if OP activates a spell speed three effect in chain, the opportunity to activate cards such as Mirror Force is now gone. There will be a response chain after the resolution of this chain, but by then it will be too late to respond to the attack.