The toughest boss in the game naturally enough. I would strongly advise the homing laser for this fight (you should hopefully have double homing stars from the previous green battle) or the flame and machine gun (as the splash damage will hit all gems for a lot of damage)
To damage this boss you need to hit the gems, hitting the boss causes no damage. From 9999 to 9000 health the boss will just stand there allowing you to damage him, after this he will become more aggressive but not overly so. Avoid him as best you can (sliding or jump hitting him will knock him down and allow you to get away). Don't get close as he'll throw you for a lot of damage. The only attack to worry about is when he sends a pillar of energy across this floor, this is easy enough to dodge however stay away from the wall as when it hits it the explosion will hit you even if you jumped the attack. Should he attack with his five fingered bombs sliding inside their range and knocking Golden Silver down should avoid them, or you can just move back.
The next phase will see the gems now revolve around the boss, this is the toughest part of the fight as he becomes more aggressive still. As before just keep avoiding him. if your laser targets the boss instead of the gems simply stop shooting for a fraction and start again as this will make it retarget the gems.
In the final section of the fight Golden Silver will retreat (On normal at least) and one of the four gems will attack you. Blue will shoot a laser down (just run/slide right or left to avoid it) red will fire off a swirling flame, time a slide right to avoid it. Green will send a homing attack towards you, wait until it is about to hit the ground then jump to avoid this (earlier and it will change direction up to hit you). The white gem attacks with a stream of machine gun fire, one easier modes this is actually harder to avoid (GS doesn't retreat on harder modes and can be used to block the attack) just do your best to get beside the gem and slide back and forth under it to avoid getting hit.