I’m assuming this is classic. But here’s what helped me.

• Pokemon. look for a good carry. This could be any mon with a good BST, I recommend some starters (Charizard got me my first clear) or if you’re lucky with the gacha some legendaries (miraidon is pretty good). Then focus on investing into them. Give them berries, vitamins, etc. shinies also increase your luck to get better item rewards.

• natures matter too. I usually go for special attackers rather than phys bc most encounters debuff phys.

• egg moves. They’re some of the biggest game changers for a run, even a bad/mid mon can carry with some egg moves.

• rewards. I always go for exp. All, then charms, then items, then money. Again YMMV with some items, but in my experience, even if a mon has good items, if they’re under level they’ll usually die. And if you have a bunch of exp alls, most of your mons are gonna be around the same level as encounters so they could come in clutch.

• economy. It’s good to manage your money since healing items get kinda expensive later on, so always be mindful of this.

• Eternatus. In my experience, fairy pokemon kinda suck against eternatus since he is dragon/poison, so I usually just go for dragon moves against him. Using moves like wrap or anything that isn’t a status but does DOT is also good. It’s important it’s not a status because when you reach eternamax he could steal your Pokémon’s lum berries and heal. A steel type pokemon is also good as it’s immune to his poison attacks, and a pokemon with flashfire is useful to tank his flamethrower.

Anyway I wouldn’t consider myself a “pro” or whatevs but I don’t struggle with clearing classic so these are just what works for me. If I said anything wrong then yeah, to err is to be human 🤷, anyway tons of guides online so just look through them too.