This stage has 50 enemies on it. All of them are level 20 promoted classes. Even if he died in the end, Donnel still brought those 50 enemies down to just six.

Donny is by far one of the fastest growing units. Late game, he should be one of your best, assuming you trained him.

What you are referring to is post-game, in which, with all classes/skills available to units and maxed levels/stats and all the time and resources in the world to dole out favoritism in amplitude, Donny is not the best, but even then he's not bad. (At least considering he came from the first generation) He ends up more along the lines of average, considering how he gets access to Despoil, Armsthrift, Sol, and Counter.

The ordinary player who doesn't have time to grind all units excessively and obsessively (or just plain can't because of the price of Reeking Boxes and lack of street/spotpass exp in higher difficulties) will get more use out of Donny than pretty much all units barring Donny's child because of the immense effect of his Aptitude ability.