If you kill a mob with an area attack with no target, the mob will continue to keep the aggro state until its dead body dissappears. This means that only M-1 mobs will attack you from mobs with M:1 aggro type.

For mobs with 1:1 aggro type like skeletons, you can kill a mob with windmill and keep the rest of the mobs in no aggro state till the dead body disappears.

Usually a dead body disappears after 15 seconds. You can extend this time to almost full 1 minute by making a single player party and change party option to "finish to anyone". The downside of this is the inconvenience of having to finish all mobs manually. (by hitting it one more time after they get killed)

There is one more thing to remember. There are dungeon rooms that spawn mobs 3 times. Once you clear such room, new mobs appear. For a short time (around 2-3 seconds) after new mobs get spawned, the game AI does not choose which mob will switch to aggro state. If you attack a mob in that period, the game AI might choose another one to switch into aggro state.