FireMan: Go to Higsby's and buy 2 Aqua Swords. Then go and fight
FireMan. It would take two hits from an Aqua Sword will immediately
kill him.

GutsMan: Make sure you have Roll and some gun chips with you (i.e.
V-Gun, Cannon, Shotgun, and Hi-Cannon). Use the gun chips first and
then when you start to lose health, use Roll. If you do not have some
of the chips, FireMan works well also.

IceMan: This guy is the easiest of them all. Simply use FireMan,
FireArm, and some strong gun chips to beat this boss.

NeedleMan: Same as IceMan only does more damage.