Anything you like. Honestly, you're a 20th-level character. It really doesn't matter what spells you use, or even what class you are. Any halfway competent 20th-level character can defeat an infinite number of level 1 warriors. Actually, even an incompetent 20th-level character can defeat a near-infinite number of level 1 warriors.

I mean, what's to stop you just sitting there and killing them all with melee attacks? 10 rounds a minute is 600 rounds an hour. 600 rounds an hour is 14,400 rounds in a day. D&D characters don't suffer from battle fatigue, and a 20th-level character's defences are far too good for a 1st-level warrior to hurt them. You should be able to get rid of all 10,000 in a day or so of steady fighting, assuming they even try to fight rather than scattering or running away once they realise they'd have a better chance against the Tarrasque. The only reason to use spells or special tactics is if you're on a schedule and want to finish things up quick.

- Saph