Kliff

with the speed boon, he'll avoid doubles with 40 base speed and still reaches 30 def using fort def/res and thats not even considering the additional buffs he can receive from his tome and teammates

if anything -def seems like a favorable bane since you might not put CC on him and it doesn't interfere with his magical matchups like -hp might

You can probably try something like this.
Make him a long lasting mage.

Swift sparrow or even close counter are ideal, but as long as it boosts speed and doesn't boost atk out of combat, it can work

I was having an in-depth look at Kliff's IVs ( https://fireemblem.gamepress.gg/q-a/lets-discuss-kliffs-atk-iv-preference if you want to look at it) and my conclusion is that he needs to be taking counterattacks for special charges, which makes -def really annoying. It's especially annoying because you can't compensate using atk ploy because that will in a reasonable number of cases deactivate his prf effect.

+spd is pretty good though, and I definitely think he's still usable with this bane. (I personally probably wouldn't because fortress def/res is so good that I'd probably fodder it every chance I got.)

I do believe ploys or other debuffs are in order because his atk stat is low. If you're putting the ploys on a teammate, fortress def seal is an option. Make sure you go with some kind of def that works on both phases.