/cmi killall command provides option to cleanup currently loaded entities by your provided criteria.
By default entities which has custom name will not get removed.
Entities in boats or minecarts will be ignored.
Range can be provided to remove in close range only.
-monsters will remove all monster type entities. This in general includes zombie, skeletons, wither and so on.
-pets will include any entity which can be tamed, like horse or wolf
-npc will includes entities with metadata of NPC usually results of plugin like Citizens
-animals removes animal type entities, like pigs or cows
-ambient entities like bat
-named any entity with custom name
[mobType] any mob type can be defined to only remove that type of entity
-f will combine monsters, pets, npc, golems, animals, ambient, vehicles into one
-lightning strikes lightning where entity got removed
-list print out entities in appropriate category.

/cmi killall – will remove all monsters
/cmi killall 60 – will remove monsters in 60 block range from you
/cmi killall zombie will remove all zombies
/cmi killall zombie skeleton will remove all zombies and skeletons
/cmi killall -list will list entities
/cmi killall -monsters -lightning will remove all monsters and will strike lightning for each removed entity at their location