 So you have 2 problems regarding drill, here are the solutions:

1. About Rocksteady, you could spawn a delay entity together with drill. The delay will hold Rocksteady or any enemy to be spawned before it's removed. Read OpenBoR Tricks Tutorial to see how to make delay entity.

 The level should be something like this:

spawn  FootSoldier
coords x z
at        0

.... (foot soldiers are spawned at this time)

group 1 1
at       0

spawn Drill
spawnscript  data/scripts/noscpos.c (see #2 below)
coords  x z
at       0

spawn delay
health  15
coords  x z
at       0

spawn Rocksteady
coords  x z
at       0

 delay is enemy therefore Rocksteady won't be spawned until delay removed itself.

2. There's a spawnscript to make drill always spawned at exact spot regardless of scrollpos. It's attached below. Place it inside data/scripts folder.

 Oh yes, this is with the assumption the stage is using direction both.