Stag Knight, Mausoleum Mage, Apothecary, Coldheart - in more or less that order, I would think. Also, your Kael's build isn't exactly great, he's super slow and low accuracy... might be good enough for farming campaign, but probably not for arena, dungeons or CB? 

I was also thinking Stag Knight first. I am at Nightmare difficulty 3rd stage in campaign but cant pass through whatsoever Catacombs of Narbuk. Arena is a nightmare too i mean i have 65k plus power in total but a 35k team obliterates me even with High Khatun is in my starting team leader. What do you suggest to build a better gear for Kael? btw CB is not bad i do 4.5m damage and dungeons meh i can make it through stage 12 right now.

It depends what do you need help with. Not all champions do the same things

My first priority is clearing nightmare campaign, what should i improve most to clear campaign easier? Zavia, Kael, Uugo and Elhain is my campaign team right now.

No. That's absolutely the wrong priority. You're not going to clear nightmare campaign until, at a minimum, several months down the line when you've got a bunch of endgame champions in endgame gear. Forget nightmare campaign for now - maybe if you're very close to the next reward you could still try to grab it, but after that, just ignore it completely. 

Your priority should be dungeon progress, as well as CB progress, while farming brutal 12.3 for xp to level up more champions. You mention 4.5 million damage on CB, but on which difficulty level? 

As for arena, well, if your Kael is that slow, it's not very surprising that you'd lose even against low power but faster teams. But you got lucky with Zavia and Stag Knight - once you have decent speeds, a team of HK, SK, Kael and Zavia could be very strong. Uugo might serve in SK's place for the time being, the important point is that AOE decrease defense. 

Replied to some of this above, but as for what I suggest for Kael: like all of your key champions, he needs boots with speed as main stat. The login reward lifesteal set has boots with speed as main stat, but I see your Kael has other boots, I guess your good boots are on someone else? Of course HK, Zavia, Stag Knight, Uugo and so on all need boots with speed main stat too, so it's normal that in the beginning you may not have enough for everyone. But your main nuker should definitely have them. Although since you have Zavia, you might be able to make her your main nuker and prioritize her over Kael for gear (for campaign farming she has the major disadvantage of being weak affinity in 12.3, though with good enough gear she could still do it, or else you use her as main nuker in arena/dungeons/CB but keep Kael as campaign farmer). 

Thanks so much for the advices I have +16 epic 4 star lifesteal boots on Zavia with 35 speed 15% atk % hp and % crit rate so Zavia is better for me on everything else other than 12.3 as you mentioned. But Kael is doing a good job on dungeons and cb as well so i am gonna farm dungeons until i get something better for Kael and meanwhile will prioratize leveling SK. I will farm more speed artifacts for arena too but its not my number one priority of course.