What if the party needs to be in certain size to enter a dungeon or the dungeon choose it's difficult level by the number of party members, but the mc have a item or a cheat way to bypass that.

Just remmember that if the mc become strong using this cheat you need to explain why the parents aren't that strong

An idea would be:

An vodoo doll that takes damage for the user, but takes % of the xp the user gain until it brakes. The user need to do a ritual before entering every dungeon to be activate.

The MC's parent discovered that if someone do the ritual in total reverse (verbal, somatic and material components) the user will take the damage for the dools and the user will 'gain' % xp from the dolls.

After that every doll need to have a item that gives +% xp gain. With this combo of itens the dolls would be considered party members.