I was kinda surprised. As Zilean, my bombs do over 1000 AP damage and I had Morellonomicon to contain yet Mundo HP regen didn't look like would be slow down by Grievous wounds. Mundo still had cosmic regen speed even being constantly bombarded by my bombs.
My build: 
Mundo build: 
In few seconds Mundo regen 2000 HP even being constantly bombarded by my AP build. Is there anything what AP can do to stop Mundo, build ?
I think since Mundo's ult is considered "regeneration" (whatever that is) instead of healing, Grievous Wounds doesn't affect it. It's even listed as such on the wiki article.
It does however affect the healing from his Q.
EDIT: Also consider that he regens 100% of his max health over 12 seconds. Even with the heal reduction per second, it's still going to be ridiculous.
I was kinda surprised. As Zilean, my bombs do over 1000 AP damage and I had Morellonomicon to contain yet Mundo HP regen didn't look like would be slow down by Grievous wounds. Mundo still had cosmic regen speed even being constantly bombarded by my bombs.
My build:
Mundo build:
In few seconds Mundo regen 2000 HP even being constantly bombarded by my AP build. Is there anything what AP can do to stop Mundo, build ?
It works, Zil just can't keep consistent damage on him for you to notice. If Zil could keep hurting him frequently like others can then you would notice it because he would get scared and run away. Anyone who can keep frequent damage piled on heal regen champs like Mundo will kill Mundo with grievous wounds (GW). Zil's bombs don't go off for a few seconds so during that time he's healing. By the time the second bomb goes off there is maybe a second left on grievous wounds before it reapplies.
TLDR, Zil can't do damage fast enough for GW to be effective on regen champs.
Gonna need some video evidence on this one Cap'n.
well heres the thing you have one source of reduced healing, where as mundo has multiple sources of healing, his ult, passive which gets stronger with more hp. ( runes) warmogs, adaptive help, spirit visage ... so unless you are doing % hp damage that lil 35% grievous wounds for 3 seconds isnt gonna do much of anything
His ultimate got buffed. So with grievous now, he's regenerating gas much as without grievous back then. You need to bait his ultimate before fighting him. Just like taric, yi, etc...
