*enemy controllers that target the invoker
*artillery that targets the invoker (he will get worried about saving himself rather than targeting other enemies)
*lurkers (the lurker grabs him and drags him away into some 'bad area' or something)
*minions, just increasing the number of creatures the PCs have to deal with at once
* combat objectives other than 'kill them all'
* terrain effects and features that limit his mobility (i know you said teleport a lot, but it's not infinite teleport all day long, after a few encounters, it's got to start wearing down)
* creatures that get a bonus save, or save bonus, to shake off his effects
* though this falls under terrain, but maybe there is a combat in an area that is under a forbiddence effect (i think that's what it's called -- it's a high level ritual that prevents teleporting into the area -- just say that it encompasses the whole room/chamber that the combat is happening in, if it's thematically appropriate)
* dispel magic type power (see wizard level 6 utility power, with errata) for an enemy to get rid of some zones
* more encounters per day
* change how you introduce a combat encounter -- are all the enemies out in the open to start with? maybe instead some waves of enemies come later from the back of the party, thus getting them surrounded
obviously, don't do all this at once or all that obviously otherwise you will be (unfairly) just targetting the invoker for doing what he should be doing. just some ideas.