Vampire

Vampires are summonable creatures that run from garlic and mirrors, and are weak when fighting vampire hunters. They will instantly transform any human character they bite into minions called Ghouls. Can be killed with a stake, garlic, holy water, or by summoning the sun. If a Vampire is summoned into a sunny, outdoor environment, he will instantly turn into ash. Vampires are scared of crosses. Similar creatures can be summoned using the words "Blood" and "Nosferatu".

With the constraints above in mind, note that vampires are immortal and thus kill everything else. Add diving mask to them if you need them under water. Can be used to clear pretty much every level of enemies.