With this build you'll have 700+AP (without baron buff) and two bonus sources of damage which scales with AP (DFG and Lichbane).
DFG allows you to nuke people down from 100% to 40% health. Your E combined with W and Lichbane's passive can deal 2K damage in ONE auto attack, and it stuns! Because of the stun you can also follow up with Q which deals 700 more damage.
This build also gives you 20% CDR, which means Q and W have less than 5 seconds of cooldown. You can spam Q in team fights which deals massive AOE damage (like Ahri's Q), and you can land a stun every 5 seconds.
Stacking magic resist doesn't mitigate much of TF's damage because
1. Lichbane deals physical damage
2. Void staff nullifies half of the MR.