The levers should be right beside the healing chambers, and are there from the start.
Leaving the minions around helps slow down General Raze when heading for a healing chamber (as would a summon and/or your creature), but other than that it doesn't matter which you go after first. If you stand near one healing chamber that is the one he will attempt to use first; block his access and he will try to make it to the other one.
Level 24 is low for that area; I think he is about level 29, so would be difficult to take down even without the minions and healing chambers.
Have you finished off Stone's Flying fortress? If you've cleared a few caves near the start of the fjords, and the path through to High Hall and the harbour, etc, Stone's fortress (north east end of the long island in the middle of the channel to the harbour) would be the next logical place to go; I entered at about level 24/25 and made it to level 29 just before the end.
