to make a unfriendly encounter for a wizard:
turn on the sprinklers
lure them into a room pre-built with a magic circle
turn off all the lights ( wizard becomes a target if he summons light and friendly fire is a pain )
have lots of innocents around
to make an encounter unfriendly for the physical people:
unstable ground, loose sand, ice, knee high water, a kids inflatable jumping gym ( great fun here )
Aoe, like grenades
choke points, make them walk a path to get to the villains with pits on either side, or walk across a plank between two buildings
multi-tiered fields of combat, makes them have to split up and focus more on the movement then the beating
now to some of the above, the nickelheads fall back to a Family Fun Center (
http://www.fun-center.com) you now have several engines of destruction in a giant building full of kids and parents, on can go hide in the giant climbing structure (think McDonalds on steroids then times 10 ) another goes to the lazer tag area and the other heads out side to the out side area.
inside you can make use of the sprinklers after some fire has been thrown, plenty of humans to make the wizards sweat, all of the people will make for a hard time running anywhere, multiple floors allow for some great chases.
outside, the miniput course will provide a great place for a fun battle as you have all sorts of odd things in the way and no good straight paths.