I ended up using Shell and Fire, too, and got it first try with that combo on my second account. My first account had no problem obtaining a Mango early on.
My 3rd account - well pbbbbtttt. I used a 10x breeding boost and a 10x hatching boost and did a bunch of speed breeding, spent way too many gems, and still no Mango. This account does not have any pure Tropic dragons to try with, but I went through every red/yellow combo I could muster. The good news is that with the speed breeding I managed to get a lot of dynamite to mine for island parts, and I got quite a few dragons that I can disenchant for Essence. I guess this account will just concentrate on building up its food stores for the next leaderboard and then bam! I can raise all those Champion dragons to at least level 10 or more.
On another note, does anyone else think of the SNL skit about "Mango" when they see/hear "Mango?"