Fun game. Kinda unbalanced though.
Marill and Cacnea are very strong and heals are overpowered. I beat Marill with no upgrades first try. You outheal any damage you take. The shell bell is great for that reason, but it's much harder to complete a run without any source of heals.
Every single run that I've won - 6 so far - has been because of the teeth item which costs 90ish and can give that much money in a single fight. It is by far the most important item.
The other items that make runs a lot easier are shell bell, x-items, and pure stat boosting items. Crit is the only buff that matters - I had a croagunk run with all poison items and upgraded moves and it still didn't deal any damage. The cubone debuff is also trash. The best build is pure damage and heals, and crits if possible.
I also have no idea how to evolve pokemon. I got Raichu by thunder stone and got Marowak and Cacturn somewhere along the way, but I have completed runs on other pokemon that didn't evolve.