If you are stuck at Slench 2A or any boss past that and keep running out of ammo or life, then just skip it and go to Celestial Archives. Yes you can do that and while you beat the archives stage, be sure to pick up four UA Expansions (that's 120 extra shots), 1 energy tank and 1 missile expansion.

Then skip Slench 3B and go to Alinos. Be sure to pick up 2 UA Expansions, (that's 60+120 extra shots) and an energy tank.

Then skip Cretaphid V3 and head to Acterra. Pick up another 2 UA Expansions, (now a total of 240 extra shots) and a missile expansion.

Skip Cretaphid V4 and head to V.D.O. While beating this final level, be sure to pick up 1 UA Expansion and 1 Missle Expansion. Now head back to Slench 2A and try again. You should now have 270 extra shots plus 198 extra life. You should find it easier.