Here is a list i made of the weaknesses of each boss including willy's castle.
For willy's castle i only list the stage and the weapon required for the boss.
Don't forget to abuse of the save feature everytime you get to a boss room or during the stage when you get to hard parts.
1st boss that should be killed is toad man, he is the easiest, all you need to do is keep shooting without charging and he will constantly jump over you and never attack.
boss/weakness weapon: (they are in the order you should fight them)
1.Toad man - Drill
2.Bright man - Toad
3.Pharaoh man - Bright (in the 1st section of this stage jump over the hole using R.Coil to get the B weapon and for the boss hold triangle for turbo shooting and he will not move for the whole fight)
4.Ring man - Pharaoh(charging the pharaoh weapon will make a huge ball with huge damage)
5.Dust man - Ring
6.Skull man - Dust
7.Dive man - Skull (after 2nd whale mid boss drop in the hole right in front of you for the W weapon, don't forget to save before dropping cause lots of spike that can kill you)
8.Drill man - Dive
B and W weapon are not necessary for progression but useful for some area
Willy castle #1
stage 1: Ring
stage 2: Dust
stage 3: Pharaoh
stage 4: Dust
Willy castle #2
stage 1: Charged mega buster
stage 2: Ring
stage 3: Boss Rush (don't use too much of your pharaoh weapon cause you will need it for final stage, minimum half of the bar)
willy's 1st form: Ring
Willy's 2nd form: Drill, you need to shoot the blue light, when you shoot the drill if you hit square a second time it will make it blow up
Final stage: Pharaoh, if you have less then half a bar of weapon grind the ennemies until you have 1/2 or 3/4 of your bar, i had 3/4 and 1/4 was left, just make sure to have more then half or your screwed if you don't.
thx for using my guide, i hope it was useful!