Yeah, we need details on the scenario that @Retreater ran because on the face of it, it doesn't make a lot of sense. Then again, it seems like the group has custom items (perhaps from a module) like a "Wand of Reverse Gravity". So, yes, being able to cast a 7th level spell multiple times per day is going to cause issues. If you have an item that you would normally be able to cast once per day at 13th level and also have every enemy show up inAre you saying the 6th level group of 7 "easily" took out a group of 25 non-mooks including multiple high level casters (so presumably 13+ level casters) without difficulty?
That's just head scratching to me! Multiple high level casters alone should be a challenge to a 6th level group - they have so many options!
Heck ONE high level caster played to the max will be very difficult for a 6th level group. For example, Improved invisibility and then some nice AoE is devastating (few groups stock see invisibility, especially all the time).
6th level PCs can be competent sure, but they don't have NEAR the shenanigans higher level groups can bring to the table!
But without details we can't offer much advice. If they actually have such things, the only option may be to take away some of the toys. Giving a wizard this kind of power and then setting up situations where it actually works is kind of like giving the fighter a tank and then wondering what's wrong. I'd talk to the players outside of game time and ask how they want to fix this.