Truvativ powerspline only tells you what crank interface it uses. We need to know what crank it is, in particular if it uses (7mm is very uncommon) an 8mm self extract, or just a plain old 8mm hex bolt. You can take a picture of it and we can tell which one it is. Once you are sure you are using the right size hex key, minimize the angle between the handle and the crank - and then use some elbow grease.

Hope it's not seized too badly.