One really easy way to do this, the 300 creature kills, AND get some good exp out of it, is to do the Minutemen radiant quests. All that you need to do is progress the Minutemen questline until right after you take the castle, and dont worry, this will NOT affect your relation with any other factions. After you take the Castle, Preston will start feeding you quests involving either clearing a prospective settlement, or doing something for an existing one. This always entails fighting enemies, and the quests are short enough that the XP just rolls in while you farm the kills. Food for thought, guys! Happy hunting!
For reference, creatures are literally anything that is not purely human anymore. That includes super mutants. Humans are raiders, settlers, triggermen, the forged, etc etc