Mordekaiser's white bar shield passive thing is impossible to fight

I played against a Mordekaiser top right? wrong I faced a spikey wall get too close you get hurt, but the worst part is, Nunu's E should do about 120 ap damage at lvl 1, but when I used it in trades, it seemed to do 20 damage (I watched the damage show after I hit him) now, the "shield" I thought would work like Yasuo's, takes some damage it all goes away I WAS DEAD WRONG it continues to go until A) it slowly depletes or B) you do enough dmg to get rid of it, but what I noticed is when he would use an ability, the shield would come back up insanely I guess the best thing I can ask for is some tips on how to fight him, since I've been seeing him more and more often (I shall smite Theeee)