The heroes could be given a new ability each at level 10.
This would be ONE user targeting, per hero. i.e. player can click the hero ability, and then a target building/enemy character. Your hero would then target that building/character until it destroys it (or is destroyed), before going on as normal under its AI control.
You could add a second target choice at level 20, a third at L30 etc.
So, a L40 hero could be assigned 4 targets, assuming it survives long enough.

