Well, I finally finished level 6 and 7 and am back to level 4. An interesting thing happened in level 6 - after the 3rd wave, the monsters started ignoring me - an apparent monster freeze problem. This made finishing the level easy. Forrest, I'll send you a rather humorous screenshot of a group of hunters and fighters gathered and just standing there - sure enough, as per John's comments, the hunters are well above the floor level. In terms of the monster freeze problem, you may have not choice but to reduce the monster count - just too many active monsters.
When it comes to the core itself, nothing is more frustrating than getting zapped off that slowly rising pillar by a compiler. My strong suggestion would be to use a monster impassable poly to keep the compilers out of this room, to populate the core room and passages with additional monsters (of course palying attention to the monster count) and to add a PB and shield recharger in that central passage off the cricular core room.
Oh, and just a reminder on level 7 to consider making those extravision power-up's invisible.
