block buffs and heal reduction are the best debuffs. decrease max hp is slow, but effective since it can't be resisted or cleansed. that said, the best counter to wythir is often to just not fight her in the first place.

She keeps showing up in 1 or 2 champ teams in classic.  She never wins but she stalemates the battle.  

How much acc should my debuffer have?

As much as humanly possible. When people build her, almost always build her with tons of resistance, high HP, high DEF. 

Better to just run a max hp reducer imo.

Its easier to ignore Wythir than to gear a champ just to battle her.  Thanks for all the info.  

How about Jack?  His A2 removes all continuous heals and replaces them with poisons.

Well Im Wythoutir so I cant help.

That may be a good call, worth trying on the annoying &%$*  lol