You mean when you have to charge the cannon against her? Once she kisses you, you can struggle from her grasp like with any other grapple move and simply fire the cannon. If you mean before charging the cannon in the normal battle, use wind magic to break free, she is too slimey to use your hands to break free.
You can move the window left and right and it should tell you the requirements of each spell. For Hell Break, max out the fire magic and the Limit Break spells.
Finally finished my True Mercy run, only to find out later the game was bugged and it would've been much easier if I just updated... Oh well, at least I can say it was a challenge.
With all that said and done, would anybody be willing to share a Genocide file? I'm not necessarily looking for a full save file, but I want to fight at the very least the final boss, and I don't really want to go through the entire game once more, I've had enough RNG fuckery for an entire life I think.
I'll share my True Mercy save file as well. I don't seem to have a 100%, probably because you need some scenes from the Genocide route as well, but every girl is unlocked and with maxed out friendship, so all their scenes can be easily viewed and can be refought in the pink bed.