Hmm, this battle seems familiar...
anywho, super buu has this in the bag, firstly due to the fact that he simply outclasses Janemba and secondly due to Janemba's ridiculous weakness. SSJ3 Goku put up a decent fight against Janemba, in fact from what I saw there wasn't that big a gap between them (Janemba mainly had the edge, it seemed, from his reality warping powers).
Bearing that in mind, Janemba will be going up against an opponent who in a weaker state was able to crush somebody stronger than SSJ3 Goku with relative ease. Whilst the said reality warping techniques of Janemba may prove a nuisance for a bit, this is ultimately one fight he cannot hope to win.
I would honestly put Janemba around Mystic Gohan level, if not slightly stronger.