Mega Man 2
Bubble Man's room is very tricky as it has spikes on its ceiling. And its filled with water, so Mega Man has that advantage of jumping higher, but you must be wary of the spikes. Bubble Man shoots his Bubble Lead if you don't initiate the attack. Shooting him will make him jump & he'll shoot at you with his Water Buster when you're at his shooting range. The best way to attack him is to move towards him & shoot, then when he jump towards you, you can fly over him & the bouncing Bubble Leads. If he jumps over you, he'll land on you so run away from him if he does that.
Bubble Man is weak against Metal Blade, which you will get if you defeat Metal Man beforehand. You can take advantage of its 8-directional shooting range.
In Dr. Wily Stage 4, you will encounter Metal Man again for a rematch. Use Metal Blades sparingly as you need it to destroy Dr. Wily Machine No. 2.
Mega Man 2: The Power Fighters
Simply, Bubble Man will trap Mega Man with a Bubble Lead and Mega Man cannot get out. Bubble Man defends with a bubble and will summon Snappers when lasting the battle too long. Shadow Blade is the best weapon to use it.
Note: Bubble Man and Flash Man are the only Robot Masters to never appear in the Mega Man cartoon.