I have a level 40 NP3 Lancelot that sees decent usage... not your kind though. With the Master ATK skill and Waver NP right before, he can get the assassin and zerker down to to ~7K each, and get Waver down to ~19K. My Stage 3 usually goes:
Round 1 - if zerker's skills haven't reset and/or if Waver's skills haven't reset (usually the case), attack waver. This is usually good because he hasn't buffed anything yet so I can usually get a good solid 15-20K on him. They usually respond by an attack (or two), and then buffing the hell out of their own zerker.
Round 2 - The money round.. Hopefully I have a buster (even better, a zerker one). Master ATK buff on Lancelot, both of Lancelot's Crit buffs, and if Waver's NP charge can put Lancelot's (or a good support Waver's) NP at 200%, even better. Now I do Waver NP, Lancelot NP, and attack (hopefully lancelot buster) on the assassin. This usually ends the round with assassin dead, waver at like 15K, and their zerker at ~7K. The best part is, with their zerker alive, Waver will almost never go def buff crazy. Maybe get one def buff off.
Round 3 - clean up.
funny part is I am running jeanne... but I honestly don't have to use her NP ever. I've learned that just healing a priority target during the second chimera, and throwing up Master Evade (usually once on Lancelot) assures that all three make it to Stage 3 no problem. I've only lost one Lancelot and it was like a turn 1 3 x attack on him by the Stage 2 chimera (at level 40 I only have like 6K health)
funny part is.. I have a screenshot of a match last night.. before I settled in on this pattern.. man there was one battle that went out 25 rounds... holy balls.. just an unlucky character loss early on and the dominoes toppled from there. But since I've settled in on this pattern I am pretty much done by turn 9-10.