Is a -atk Kliff still good to use?

-Atk is ideal. Give him close counter if you plan to +10 him. Otherwise, stick with his base kit.

An attack bane and a speed boon might just be his best IV set, in all honesty. It best capitalizes on his tome and tanking power. And for CC vs Fort Def/Res, he'll have 33 def/res (39 if you run CD3) with CC, and 39 def/res if he uses Fort Def/Res, which you can bolster with CD3 or DD3 in his seal. Either way should be fine.

thats pretty optimal I'd say. it will be better with CC since there is no need to lower his attack any further than it already is with fortress def/res. if you like him, then its worth the investment

He’ll pretty much be LA!Lyn if u decide to give him Close Counter

+spd is really good and my preference.

A lot of people are saying -atk is optimal, and I think they need to show why. I went through the numbers, and you can find my conclusion at the link below. I think -atk is not to be preferred. Even if you end up disagreeing with me, I think you'll find it helpful for understanding what to do with him.
https://fireemblem.gamepress.gg/q-a/lets-discuss-kliffs-atk-iv-preference

I suggest against close counter for reasons I explained in a comment at the bottom of that link. I do admit that you have one of the best IVs for it.

For +spd -atk, special spiral ignis/glacies is the best I know how to recommend for him.