kog'maw is balanced around guinsoos but cant build mortal reminder

kog'maw is balanced around guinsoo's rageblade but ever since they changed it to be 15% mpen/armor pen and incorporated the dissolve/whisper passives which don't stack with either dedicated penetration item kog'maw hasn't found a solution to deal with champions that heal out of the ass. I literally can't build the mortal reminder because I'll only get half (or even less than half) of its effects