So, I'm running a dnd 4e campaign, and one of the characters in the party is a level 12 monk. He keeps killing the monsters I throw against him, and it is getting difficult to make encounters challenging. What are the best types of monsters to throw against a monk in 4e?
Monsters that cause:
will always extend your round count provided the player does not save against the effect.
Status effects will always be your friend, Not just ham-fisted heavy hitters.
If you want your monk to not wipe the field as quickly, target him with a Weakened and/or Restrained to halve damage, an Immobile, Prone, Slow, or Grabbed to take away some movement, and Dominate*, Stun, and Daze to remove actions they can take during their turn. Of course, the monster of your choosing should be powerful enough to have a fair chance of these effects sticking.
Unless you really want to fight the monk's proverbial "fire" with "fire", use a ranged creature or caster to inflict these effects. I suggest a reach of two or three for a single melee monster to go against most Monks, especially if this Reach is granted by size alone. A medium sized creature with this reach should probably have a lot of abilities that allow them to shift around the field before or after striking the target in order to prolong their life.
As an aside, I highly suggest incorporating some party dividing tactics to spread the Monk's attention. There's nothing like being caught between two lines of archers that refuse to converge.
If all else fails, use things that Fly! After all, if it's out of range, it's out of range.
(Ranged melee with fly? Ugh.)
(*If you Dominate the Monk, you can still use the At-Wills, as best I can recall, which can definitely throw a wrench into his plans. This is certainly the case if the Healer/Leader is suddenly in danger of being surrounded and pounded. Also, please make sure your party is okay with the use of "Dominate," as many players in my personal experience find the use of the status effect very undesirable.)
In summary, to challenge your monk, use flying ranged melee strikers or flying controllers that target the Monk's weakest defense.
Monks primarily target Fortitude or reflex. Monsters that have high fort, ref, or both would present greater challenges to the monk. Likewise many of his abilities are about hitting enemies in a blast or moving and hitting lots of enemies monsters that prone, grapple, and punish moving away from them (basically monster versions of defenders) combined with powerful ranged artillery monsters could help you put the beatdown on him. In general monsters from the far realm have proven tough for my wife's monk to hit.