@all_starsupes: Janemba has regen and can teleport. Janemba can also open small portals which he usually uses to punck, kick or fire blasts through. He can also use it for defensive purposes, like porting an attack away from himself.
@JThree47605: I'm sorry but you must be joking? DD has solo'ed the JLA, he's slapped about Cyborg Supes, Green Lanterns, Flashes, WW, MM etc. The list is endless, he would grab Janemba before he realises what is going on and relentlessly pound the crap out of him.
This is a stalemate. Janemba can easily take off and remain airborne while he shoots blasts at Doomsday who will withstand them all. There's no winner. If they decide to trade blows then Doomsday would win due to being physically more powerful.