Start with boots to move faster to your enemies, attack then with E but be careful about your health waste. With Hextech revolver you have some spell vamp, then use q as much as possible to recover your HP, remembering to attack distant enemies with E. Complete your Will Of The Ancients before your boots if you can. Now you have more spell vamp. Rylai's Crystal Scepter grants you HP and slow your opponents, Mordekaiser is a low champion then it's grant you next kills. Rabadons to boost your AP to more damage, and the last itens grants you more defense(armor and magic resist) at the same time gives you AP, turning Mordekaiser the number 1.