ludens echo gives you a burst of damge
Deathfire grasp gave you a burst of damage
both scaled with AP
DFG scaled with enemy hp
Ludens gives you heavy flat damage or just has a high ratio.
The difference is that echo is healthier with a set value for the most part.
Ludens also deals more damage depending on how many opponents are nearby
Ludens replaces DFG but only indirectly
DFG was not healthy
Ludens is not much healthier but it doesn't take much to be healthier than dfg
There are 2 ways to deal with tanks as AP or AD
first you can just stack a shit ton of the stat
second you can get penetration
ad can also get crits
AP has options like this as well
AP stacks higher than AD, you can get like.. what 600 AD with an impractical build
You can get 1000 AP or more with a workable build
And then AP tends to do a lot of burst damage
Lastly it is not the mages job to "deal with tanks" or "deal with" anyone. Mages deal damage, mages give no fucks who they deal it to or why, mages don't care about any of this, they just nuke the shit out of everything
also if you are talking about fighters when you say tank they are supposed to survive well against AP.
right now does 0 damage to tanks. Deathfire grasp was an item designed to help mages deal with tanks, but was instead abused to obliterate squishes instead.
is not enough when mages have to rotate spells on cooldown. And the flat magic pen on 
