500k/min is too low. That's 5.00e5/min, which to put into perspective is an axe-thrower's single-hit territory, once you've got sufficient upgrades.
Focus on spamming 2 different units that can buff each other, with just enough of the other elemental units to keep the other 2 elements applied to it for reactions.
And consider prestiging to unlock more points/better skills.
