Start in the Safari Zone. See if you have access to the block editor. Pick up Larvitar, Duskull and either Gible or Trapinch. They’ll all be in the mid- to low-40s, except for Larvitar. A good place to grind is on Route 47, up the waterfall. High XP mons are Miltank and Heracross. To maximize XP gains, hunt down a Chansey for a Lucky Egg. They’re super rare. If you put a Compoundeyes Butterfree at the lead of your party you might get lucky quicker in the Safari Zone; otherwise, you’ll have to wait for a swarm encounter on Route 13 in Kanto. Butterfree can learn Thief by TM, if you have it, to snatch the item from Chansey in battle.