Final Cut

Charging final cut increases the damage it does, the damage bonus it gives, and the hp it uses, so it's almost always in your best interest to charge it fully. When you hit a monster with it fully charged, it gives you 2x damage on your non-mirrored hits, not +200%. The way to use final cut in general is to charge it while standing on top of a mob or with a bunch of monsters attacking you then release it just as you get hit so you get that second of invincibility to use a potion. If you get missed, then it should be fine. If you get hit, autohp will heal if you if you have it, but if you don't have it, your FC will simply stop and say you don't have enough hp to use this skill. If you don't have autohp, just keep trying to do that until you get a miss, shouldn't take too long because thieves have really good avoid. It's really easy once you get the hang of it. There's a point you get enough hp and/or enough damage on it to just simply use it though without worrying about dying. At zakum, you can final cut straight across it with maxed final cut, or if your final cut isn't maxed, chains backward and walk into it with dark sight on and FC out, assuming you have ADS, and make sure to chains immediately if ADS works, use a potion if it doesn't so you don't kill yourself. You don't want to do this with chaos zakum though because it'll dispel your dark sight while you're on top of it. At zakum arms and ckpq, the bosses aren't lethal, so you can just do what I mentioned up there. At horntail, there are plenty of ledges you can use it on, it doesn't rush forward if there's no ground in front of you.