Stop wasting your MP on Thundaga and maybe you'll find out that THM doesn't run out as quickly as you think.

If THM got an MP reduction for combos, they would be even more dangerous. As long as you can learn to not run around like a headless chicken, use Resonance when casting when things are out of range or to change position to avoid AoE, you'll be fine.

THM is great for NMs and things that just need to die, and much like BLM, less useful for general mobs.

Again, stop wasting your MP on Thundaga.