How to Fix DFG Without Removing it.
Just for clarity: The goal of DFG is to give burst mages a chance in hell vs health stacking. The problem is that since it scales as a percentage of total health it's also very effective against squishies, essentially just giving them another hard hitting spell.
Solution: Make DFG deal X% of bonus health instead of total health.
It really wouldn't add any burst against squishies aside from the AP you get from it anyway, which you can tune as needed.