Switch to Sundano and force every other hunter not to attack. Do not attack Frankenstein. Make sure your MP bar is high (attack some of the experiments to raise it if needed) and back away from Frankenstein. If no one is provoking him, his arms will slump and he will raise an electrical generator over his head and eventually throw it. While it is still over his head, he is completely vulnerable to attack. If you can see his shoulders slumping,
immediately summon the Forest Spirit and the lion should eventually attack the vulnerable monster, knocking him down and dealing great amounts of damage. Followed with some combo attacks, you will severely deplete his HP. Before he gets up, back off quickly or else he will attack you. Wait until his shoulders slump
and repeat the summoning and the combos. He should die quickly.