Valk, Mercy, and whatever else you can fit at your level of progression that uses mostly blue, possibly some, yellow, for damage. Valk/Mercy feed each other, which makes it very easy to cap out your souls - you’ll only need 3 casts with a single traited level 18 valk/4 level 10 magic kingdoms (without any stars needed). The other spots colors can even use non blue/yellow (though purple is kinda hard denied) and you still have your valk/mercy core for board control, getting skulls on deck and getting other manas through cascades, but will be slower to finish overall.
Valk/Valk/Rowanne/Mercy was my favorite speed clear/souls team before mab was added - mercy fills valks, valk 1 fills rowanne, cast rowanne, valk 2 fills rowanne, cast for game. Later, mab replaced rowanne, and now I use Valk/Mab/Rowanne/Mercy because they form a direct chain that usually wins the battle with one cast of each rather than needing to loop. That method doesn’t cap souls, but is more still more efficienct on both traitstones and souls when one cast of valk starts taking you over halfway to the soul generation cap anyway. I’m not sure if you guys have mab yet, though, and rowanne works best when she can two-shot the opposition, so its usually a no-go before you have leveled your armor kingdoms. If not, Templar/Valk/Rowanne/Mercy is a solid variant, albeit slightly slower for me.
One last thing - if you haven’t leveled all four magic kingdoms to level 10 yet, I would recommend doing so before attempting to heavily and repeatedly “farm” souls. With a level 18 valk, you’ll need about half as many casts to cap with the kingdoms leveled as opposed to without, 3 casts as opposed to 5, which can turn out to be a lot of unnecessary time spent for the same results depending on when you commit to farming. (With some 5 star magics and a level 20 valk, you can cap in two casts, but you arent going to get there without heavy farming in the first place.) Until then, other sources of souls like getting new challenge stars at high difficulty are probably a better bet. As others said, getting a good armor is also important.