Don't allow high-powered characters. My last super-heroic game had a 225 point limit, and if I ran a fantasy campaign, it would have a 150 or 175 point limit.

There is no math at all in the Hero System, just arithmetic. If you have mastered sixth grade arithmetic, there's no problem with it at all. (The biggest knock on the game is that some people can't handle fractions on the sixth grade level.)

Any power can be controlled with the right Limitation. Just tell people that you reserve the right to add plot-inspired Limitations, which will give them spare points to put elsewhere. For a fantasy campaign, I would disallow long-distance senses (including ESP) without major limitations. I prefer Not in Player's Control, which allows the power except when the GM decides it will break the scenario.

Consider the powers they want, based on your scenario. If they have super-powerful one-target powers, throw hordes at them. If they have wide area effects, use individual powerful bad guys with large defenses.

A large monster with lots of STUN and BODY and Damage Reduction fixes many problems. Anyone can damage it; nobody can kill it quickly.

And when their power makes a battle trivial, let it. Then assume that bad guys can do the same thing, and be ready for next week, with monsters that require different tactics.

Oh, and if somebody builds a super-powerful physical fighter, then a mage with Desolidification Usable Against Others will take him out of the fight.