I always die in less than 10 seconds until I just spam attacks and not move, lasted 48s. Spawn rate could be lesser at start and gradually increase over time for difficulty scaling.
About the combos, the mechanic sound interesting. But I do not notice any if I did any combo. Maybe a visual cue and a unique cast sound effect would be nice.
The pixel arts are lovely and looks professionally made. You did great, keep it up.