A level 15-16 Valk should have 4 magic, meaning untraited and unbuffed, you’ll need 9 casts of her and the four kills (or 10 casts) to hit thencap. A surviving valk necromancy only needs six casts and the four kills to cap, providing they survive the battle. I’d highly recommend getting necromancy on Valk as one of your first traits. It is slightly cheaper to get Necromancy on Zombie and Flesh Golem, but these are harder to make work in a Valk group. Two necromancy traits also stack (even with two valks), letting you reach the cap that much faster.
I recommend leaving the mass soul farming until you have a level 18 valk and all four magic kingdoms to level 10 - this gives you 9 magic, which is enough to cap in 3 casts with 1 necromancy troop (valk), or two if you have two surviving necromancy troops, which is about the fastest you can get. Theres also what most of us did for the majority of the game - stick Valk in every single team you use pretty much no matter what for a long loong time. Arena and Treasure Hunt could also be decent options at your level, even if you have to farm the maps.
A decent lower level team (if you’ve finished wild plains) is Valk/Soothsayer/Sunweaver/xx in some order - Valk in the front with yellow red banner means 2-3 matches to fill for the first time and feed Sunweaver, who buffs Valk and Valk then only requires one match to recharge. Soothsayer also works up front (still yellow/red, just loop cast soothsayer as much as possible to get more magic faster), and the third troop can be whatever to cover the missing color - Skeleton in front works nicely, Boar Rider in the back or second is also good. Tyri could also work well. I’ve also used Zombie in the front here - safer and more necromancy bonus, but since he eats skulls, you could have a hard time cleaning up. Depending on who you use, they should be enough to get you a decent amount of souls you play instead of extending the fight just to farm. Theres also the classic of Treant/Alch/Valk/Banshee, but this will tend to be slower as the main reason to cast valk is to set up for Banshee, which sometimes takes some waiting or clearing to do.