Re-adding DFG - but ONLY vs tanks

i think it is a little over the top, when a very fed mage cant kill a "normal" Mundo at all. So my suggestion is:

Deathfire Grasp re-added

3400 g 100 AP UNIQUE active: for 15 seconds:

• deal 1 % extra damage for every 50 max HP difference, capping at 1000 HP (20 % increased damage) to target enemy
• ignore 45 % of targets BONUS Magic Resist for 15 seconds

> This item would not make a 10k gold mage kill a 10k gold full tank by any means. If you gor example go DFG after morello/Deathcap, you sacrifice Void staff, which give a large boost in damage vcs everyone. DFG should basically be an item you can buy when you are very fed as an APC, so you acutally can kill the enemy tank - and you would sacrifice another AP item, for example Ludens (extra AOE) and Rylais (utility).

The reason behind this: I was a fed Syndra, 11 kills at 20 minutes. I got solokilled by a full HP mundo, and i hit him with 2 balls, a W, my stun (E), and a 5 or 6 balls ulti, and anither ball before i died. The mundo had 4-2 and 50 less farm, and like 5 towers down. I think when i did so well, i should be able to 1v1 a not very fed mundo. The only thing i lacked was Ignite (just used it), and he survived with 25 % HP, so i might barely have killed him. But still, i think the difficutly to deal with tanks are a litte OTT right now.