I have a player in my group who is playing a 6th level wizard who has specialized in conjuring (specifically, summon monster for aiding the party). His abilities have become very effective in the group's battles, to the point that I've had a couple of "big bads" go down quite easily because of what he can summon for aid. I don't mind this, particularly, but I am hoping to make some changes to upcoming battles that could be a challenge for this group.
I don't want to just throw more enemies at them because, for all of their capabilities, they are not capable of soaking a lot of damage. What I'm looking for is something that will challenge their abilities (not negate them entirely), not just throw more damage at them.
In the group there is this 6th level wizard, as well as a ranger, a binder, an inquisitor and a bard (all 6th level).
Any suggestions would be welcome.
Thanks!