How are any of them defeating Super Janemba? It required a fusion to defeat him and Gogeta is more powerful than any of the other fighters here.
Super Jenemba is slightly weaker than Super Buu. Both Gotenks and Gohan can stomp him.
I'm curious on how you figure he's only slightly weaker than Super Buu? Goku in SSJ3 never really did much against Super Buu, but he distracted Super Buutenks momentarily while Gohan attempted to find the Potara earrings. Once Super Janemba became serious, he dropped Goku like a fly.