@ Scarfwynaut, really the only way you're 100% countering Breloom with Amoonguss is assuming that something else is already put to sleep, meaning that you can either Clear Smog away the boosts or smack it with Hidden Power Ice / Clear Smog. (Is Technician released yet? If so,then Spore / Stun Spore also work.)
Also tbh I played DW a lot before this (and the current metagame is shaping up to be a lot like DW), and I always used a spread on Amoonguss of 252 HP / 36 Def / 220 SpD with a Bold Nature. The only way that Keldeo is beating you anyways is possibly a Choice Specs HP Ice, but even then you can just switch back out to a mon that can take it, meaning you really only have to take ~30%. Either way, I think that spread is best because you can still take all the special attacks that Amoonguss should be tanking (e.g. Volt Switch, U-Turn, etc.). +6 Serperior Leaf Storm does something outrageous like 40% lol. I mean, you're not really going to be tanking a Fire Blast or anything, so there's no point in consolidating your physical defense when you could spread it out and still have great defenses on both sides.