This post recommends psychological warfare and should be taken as an anecdotal observation, but...
I find that DOT's frighten Mordekaiser players. Even the ones with a zillion HP will tend to hesitate when they see their white bar ticking down and poison darts gently caressing their green one. THey will tend to at least back off long enough to call in the artillery, or if you're really cunning, into a mushroom that will give them third thoughts.
You may not kill him, but you'll give him pause. Granted, if he's got his head on straight enough to know you don't have a big nuke waiting in the wings, you'll just have to eat a cone and Move Quick your ass out of there, but that's Teemo's lot. You're not going to kill him unless he's coming out of the wrong end of a team fight.
As a team, if he's your last target alive, uhm... stun him and hit him in the face? If he's not, you've really got better things to do than plink darts off his metal hide. Try and skirt around his relatively short range, take advantage of your camoflage to let his squishy innards (i.e. the rest of his team) get in range.