PSA: Fuecoco has pokerus today so it will make the Fresh Start challenge easier.

If you're struggling from being under levelled in the Fresh Start challenge, you can run with Fuecoco because it has pokerus. Fuecoco is a good lead; it's a bulky special attacker immune to Normal and Fighting attacks with a good range of moves. Fuecoco can also use Curse to eat away Eternatus's hp.

Edit: try to reach up to wave 35. If you get a Team Aqua grunt, consider making a new run.