Napalm Man's stage is a toughie, but it's certainly not the hardest thing you'll encounter in the game.
The tiger bots are tough to kill if you let them get on the attack. During the first part of the level, always have a fully-charged Mega Buster shot to fire at the next one you see. One fully-charged shot will kill one instantly.
There are numerous new threats throughout the stage, including giant shielded robots that fire missiles out of their bellies. These bots are only vulnerable in their head, and only when it's not covered, so just rapid-fire their heads and you should be all right.
The Beat Letter is on a ledge a ways through the stage. It's not too difficult to get, and you should be able to get it without any tips.
Napalm Man himself, despite his somewhat menacing appearance, is a wimp. He mostly stays on the ground, only occasionally hopping, and he only has two attacks - firing volleys of three straight-firing missiles, and volleys of two close-range bombs. If you stay far away, the bombs aren't even a threat, anf the missiles are easily avoided by jumping. He's weak to the Crystal Eye, but he's just as easy to take out with fully-charged shots from your Mega Buster.