Dungeons in sets of five levels with group bosses

Split each dungeon into sets of five levels, for example 1 to 5 and 6 to 10, where the last level of a set is a main boss that needs two or three players. Higher sets, such as 11 to 15 and 16 to 20, need three or four players with a minimum level. Each main boss would have its own player limit and minimum player level.