It is very easy to reach stage 500+ on first playthrough, but you have to level your nymphs to level 10 for the super damage boost and level 10 skills.
Dragon and Snowfox are the two I focused on at first. Evolve both to stage 1 or 2 and they oneshot everything all the way up to stage 501, where the monster HP gets a x10 multiplier from the previous stage. Can still go on past there, they just need 2 hits now.

This is an idle game, so some patience is needed - or you can buy the gold boosters and things go wayyyyyyyyyyyy faster. The 7 day boost is well worth it, and it lasts through rebirthing. If you are absolutely determined to not spend anything, then the 1 hour boost is suggested on a fresh start/new rebirth.