I don't use Mugen or anything, but I heard lots of people saying that nightmare ssj3 broly is one pain in the *** to beat...
hmm...that broly is good but not insane.i say super ken since he can kill you with one hadouken.oh the kraken is prob a good and hard boss too.
Kool Aid man. I ain't freaking joking either, you can be fighting him, and then all the sudden you have barely no health left. He fights sneaky. >_>;
I've seen some very disturbing Evil Ryu and Evil Ken doubles that don't even give their opponents a chance to strike back.
You can always try the M.U.G.E.N Light from Death Note. I'm not sure how he works but you can K.O your opponent pretty quickly.