As a new player (< 1 month) making the slow slog to Silver I (So I can complete all of the missions past that that I am WELL past), I feel your pain.

I got lucky and pulled a Miscreated Monster that I'm currently running with a Shield set to counter the ever-present speed nuke teams in the Bronze brackets. Somehow I pulled legendary Errol, but MM is just so much better for me at this stage of the game that I put the meager resources at my disposal to get him to Rank 6.

I typically find that a rank 5 or lower Kael in a non-synergistic team is an easy win. The rank 6 fully built Kaels I avoid like the plague though (unless someone puts him solo on defense). I routinely see teams that are running two Rank 6 Kaels. Have to admit it makes me a little frustrated because I won't be able to counter that any time soon. 

Long-term I'm sure there are better options. I'm still learning the game, but for now if you have a decent HP champ, gear it up with shield set and boost it's HP as much as possible because that shield is based on the HP of the champ wearing it, NOT the champs receiving it. That SHOULD get you through the speed nuke as long as they aren't running Warmaiden to throw that mean debuff on your team before Kael can set off his AOE nuke.

If you have a Force affinity attack champ, unleash them on Kael.

I have been playing a little over a month.  I have a 6* fully ascended Kael.  He can be beaten but its usually a team with bettery synergy or several epic/legendary heroes. 

Best way to counter is speed, you just need to get your nuke off before they do. Early Como speed team would be spirithost lead, diabolist for turn meter and as your quickest champ, warmaiden to AOE defence down then your starter to finish off. With enough speed on your champs that lineup will almost certainly get you to G1.

There are a number of champs that can neutralise kael, typically in early game you’ll encounter arena nukers such as kael who hit first and hit hard. You ideally want to build an arena team full of defence based champs, who apply attack down or def up before their kael hits you, you’ll be able to to soak his attack and then return the favour. 

My early game team was: kael, bellower, apothecary and warmaiden.

I made apothecary 200+ speed so he could apply his turn meter to my bellower, I gave bellower  High speed and attack. I didn’t worry he was squishy I just wanted him to apply his a3 which applied defence down and attack down. Then came warmaiden with her attack up, the. Kael nuked the living day lights out of them. But elhain works well too

 Then came warmaiden with her attack up