Mordekaiser is a LANE BULLY so play aggressive if it is a champ that you can get to if not play back and just farm up under turret or use your E.
When there is an opportunity to engage go all out on them, usually the opposing player has not been against many Mordekaisers so they will not have seen his full damage potential leaving you with a good chance of them misplaying or having to flash, this is definetly worth all the health you lost (if you did lose any).
Always max "W" this is where the heal comes from and when maxed huge health is given back to you so you will win trades whether the person is a BURST CHAMPION or damage over time champion you can just heal it up so don't worry if you get low and get ganked it heals depending on the amount of champs around (maxing at 2 UNITS don't worry this is well over enough people to heal from about 50% to 100% healing pretty much)
UNITS : Champs, minions, monsters etc.
BURST : A champion that deals a lot of damage in a short period of time
LANE BULLY : Someone who is very strong within laning phase.