Army of Ruin

Despite the wording, I believe this achievement actually requires you to use your skill 15 times in a single game, and isn't cumulative across all runs of stages. This would be pretty tight on any of the lowest difficulty stages, but as soon as you play on the second difficulty where there are 15 waves, there's plenty of time. Just make a conscious effort to press  whenever your skill is charged, and this achievement is easy enough to do.