How to foil single-target focus?

[Mephistopheles]

One of my DMs makes judicious use of difficult terrain attacks and lurkers to hamper our coordination; not all the time but just often enough to keep us guessing.

Area attacks that create difficult terrain for the characters but not the monsters partly eliminate the ability of the characters to shift into positions they want to be in.

Lurkers are especially good: our DM enjoys dropping them on us and we enjoy the way they can totally change the way you thought a combat was going to go. Even though we've learned to look out for them and we scan the area when entering combat you're never quite sure if your back line is safe; if your perception rolls were good and you see nothing they might not have been quite good enough - although you learn to guesstimate pretty well - but worse than that is when your perception rolls are obvious failures because then you try to be prepared for the appearance of a lurker that may or may not be there, and that can keep you from committing to the enemies you can see.

 

[ExploderWizard]

I don't have much experience with 4E from the DM side of the screen but there are some key concepts that can be applied to any edition.
One thing that helps with splitting party focus is having multiple opponents with the following qualities:

1) An amount of damage, and/or nasty side effects thats hard to ignore. The damage/effect has to be very dangerous relative to the characters in the span of one round or two at the most.

2) Defenses that are not to difficult to overcome. If just hitting the things is a PITA then of course the PC's need to gang up to have a slight chance of putting at least one down.

3) A somewhat low pool of hit points. Overall, monsters in 4E have a lot of hitpoints which really does encourage the single target focus method. Have the critters with a HP pool that even a non-striker can put down in 1-2 rounds.


So if PC's see that the monsters: A- are a real dangerous threat short term, B- are kind of easy to hit, and C- don't take forever to grind down even with a single PC, then splitting focus becomes the smart way to combat the enemy.