To build a caster killer...

[Kane0]

Another vote for full Shadow Monk w/ Mage Slayer feat.

- Armed and armored at all times
- Lots of attacks to break concentration
- Speed bonus, bonus action dash and shadow step to close distance, later expands to other surfaces
- Darkness to counter 'target/creature you can see' spells, Silence to ruin the majority of casters.
- Spammable Stunning strike to shut down casting or getting away
- Evasion to avoid blasty spells
- Pass Without Trace and Cloak of Shadows for stealth
- Stillness of mind to end control spells
- Diamond sould to make any sort of save later on
- Not useless against non-mage threats
- Not a caster themselves (because where's the fun in that?)

Lucky, Mobile and Sentinel are great additions for more mobility, survivabiliy and lock-down potential. Tough and Healer are also good if you're in need of some extra HP.

Then all you need to do is pick a good race. Yuan-ti is excellent, Gnome is hilarious, Elf has the 'correct' stats, Halfling is lucky, etc.