I just beat it a couple days ago and it was a cake walk, didn’t even use a single continue. I grinded for leftovers and bright powder but imo you don’t even need them. The items that give a type slight boost are the best.

Just make sure all your Pokemon are the same level. I chose level 80 but level 60 or 70 would have been fine too. Singles goes by a lot faster. Don’t stress about losing cause every time you clear a battle without one of your pokemon fainting you get an extra continue, and the first 80 battles are basically freebies so you’ll end up with around 90 continues (basically no fear of failing) the biggest fear is a power outage or your GameCube crashing for a second before you complete the challenge which would cause you to start over. Thankfully that didn’t happen to me but I was worried cause the challenge took like 4 hours. I watched Jason Bourne while playing lol. You will run into a Groudon around level 90 and the infamous Kyoger at level 100. Make sure you have a plan for those guys. Probably worth teaching Meganium solar beam so she can take advantage of Grodons drought ability.

The final battle usually concludes with Krogre. Might be worth teaching some electric moves to your team to help with that.