First off,
Scarfed Raptor, thanks! I'll take you up on that after this post! Now, Damage calcs of belly drum azumarill *cue dramatic music*
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 252 HP / 0 Def Roserade: 118-140 (70.6 - 83.8%) -- guaranteed 2HKO after Black Sludge recovery
Nothing else is really worth putting down, as it all 1HKOs, very harshly (fun points for ice punch being able to do +400% damage :P).
So basically,
if they get up the belly drum before roserade comes out on the field,
you're done, period. you eat the first aqua jet on the switch, then the 2nd one drops you, game over. This is the worst case scenario.
However, if you do come in same turn as the belly drum, then any grass stab you have will 1HKO them at their 50-56% HP no matter what, or force the switch, which would be pretty crushing for azumarill regardless.
Conclusion: The belly drum set is the more dangerous game, for both roserade and azumarill. A burned azumarill does give you a better chance at life, worth noting, but earthquake and ice punch still murder your face off.
Some other random calcs with gengar:
252 SpA Gengar Focus Blast vs. 252 HP / 0 SpD Roserade: 33-39 (19.7 - 23.3%) -- possible 6HKO after Black Sludge recovery
252 SpA Gengar Shadow Ball vs. 252 HP / 0 SpD Roserade: 67-79 (40.1 - 47.3%) -- guaranteed 3HKO after Black Sludge recovery
252 SpA Gengar Sludge Bomb vs. 252 HP / 0 SpD Roserade: 75-88 (44.9 - 52.6%) -- guaranteed 3HKO after Black Sludge recovery
252 SpA Gengar Hidden Power Ice vs. 252 HP / 0 SpD Roserade: 68-80 (40.7 - 47.9%) -- guaranteed 3HKO after Black Sludge recovery
28 SpA Roserade Shadow Ball vs. 4 HP / 0 SpD Gengar: 96-114 (70.5 - 83.8%) -- guaranteed 2HKO after Black Sludge recovery
252 SpA Roserade Shadow Ball vs. 4 HP / 0 SpD Gengar: 112-134 (82.3 - 98.5%) -- guaranteed 2HKO after Black Sludge recovery
Analysis: While mega gengar will almost certainly rip roserade to pieces, Roserade handles all of regular gengar's tricks pretty nicely, and carrying shadow ball, you do solid damage even with minimum investment. the sub-disable set is a bother, but prediction does help. You break subs no problem, and you can powder in response to the disable.
Anyway, not too much more to say at the moment. Could list a bunch of shadow ball damage calcs, but I think I've done just about enough typing for now. leaf storm is better on the alakazam switch in than shadow ball. leaf storm only has 18.8% chance of 1HKO on alakazam, but you're going to switch out regardless or you just fall over and die to psychic/psyshock. If you have someone else to absorb status, it might be worth having technician w/hp fire on this set, as you do 1HKO 252 HP/0 Sp. def scizor while living through max power bullet punch (adamant nature, technician, choice band, 252 atk does 71-85% damage to the bulky roserade set).
And that's enough of me typing. I'll step out for a few minutes, return, and add Scarfed Raptor. I'll pass you a nice phantump or a spikes/powder roselia in exchange if you'd like. I have. A LOT. of both.