Thats what Morde's shield is for, he can take the beating that other glass cannons can't, in most team fights, minions or monsters are near or close, you can put creeping blades on one of your minions to feed your shield, or even your tank who will be near the enemy champs, also feeding your shield. Think of it as a glass cannon with a protective coating.

Sorry for downvote, but Mordekaiser lacks the CC to be a good glass cannon. Good mages like Malzahar, Veigar, Annie, Anivia all have a stun or suppress that let you get off your full combo of spells. If you build Mordekaiser AP, then you need survivability as well, or you will be focussed down so hard you won't have time to cast a single spell before you die. (At least on high ELO)