Make/use whatever you want, adjust the stats so that it's primary offense and defense (athletics, weapon, w/e) are on par with the average for your party, and don't run it with full consequences. (Read over the section on opponent importance to the story, where it talks about what consequences an enemy might take at what levels).
In high powered campaigns, you can actually blow through the entire consequence scale pretty quickly. When someone has a 14 targeting roll, exceeds their opponent's defense by 10-ish, and then rolls like 24 damage into stress - well consequence boxes kind of go poof.
But when you're doing your best to pump out a couple of spells with a targeting roll that might not even hit, the difference between an opponent with one mild consequence or an opponent with even just one mild and one moderate can literally swing the fight.
The only thing you really need to avoid is something that characters might not have access to defeating, like ghosts, creatures weak only to earth magic (none in the books, but still, it'd be tough), and so on.
Also, if they are having a really tough time, then try running a few non-combat scenes where their aspects can earn them some fate points before a battle. Most wizards go into a battle with few fate points, and a higher level wizard makes up for this by defaulting to magical defenses (the standard one enchanted item block, one enchanted item armor set up)... but at lower refresh, your spellcasters are probably burning refresh and focus items just to do their primary function at a decent capacity. So they are truly glass cannons - probably really no good defense skill, no fate points, and no magic protection. Walking into a fight with even 2 more fate points could swing the battle for them.
