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.
That's the best I've got. I'm sure more experienced players can add to that, but I see most comments revolve around using champs that you may not have access to or the resources to gear up and ascend for a while.