I'm sure it is tempting, but Giga Impact's 90 accuracy could spell your doom if it misfires at the wrong time, since slaking would suffer from truant before getting the chance to attack again. I don't know-- maybe you're into that kind of risk-- but from the experiences I've had, Choice Band Slaking kills near everything with return anyway (save for defensive pokemon like Cresselia). The risk of doing nothing for two turns (true that you can switch if Slaking misses with Giga Impact, but still) is harsh enough that I wouldn't recommend it. Brilliant; I wish I'd thought of it. I'll definitely give it a try if I ever decide to take the team on another go through the tower. Best of luck on your tower run, Carl.