In the (very) long term, no troop should be a waste of souls - in 108 and/or beyond there’ll be some benefit to levelling them all (again) - not just the ones you’re using.
But for now - sound advice on prioritising!
Just an update on how the team has been doing. Its capable of wiping matches incredibly quickly while being able to generate maps and gold on the side. Very happy with it 
Subbing out Slime for Sunweaver does give it much more survivability, but it tends to break the flow I get going. With Slime, Shadow Dragon and Venoxia, I can pick and choose gem generation on the board, potentially keeping my chain going. Sunweaver in takes away one form of purple generation and provides staying/skull power. While there’s no real flow to how my team kicks off, giving it a bit of thought and mixing and matching abilities during the match actually makes this team more versatile.
Then again, I still lost a match recently against a level 600+, so maybe I’ll have to start switching it up in the higher levels.
The Alchemist will definitely help this situation. He feeds off the brown mana and you can also get rid of the worms’ brown. Valkyrie is worth having too, since she’ll feed off the yellow and can also get rid of the worms’ brown. But control teams are lousy on defense because they require brains to operate, so you might want a “dumb” solution for your defenders.
My favorite control team is: Eternal Flame, Valkyrie, Alchemist, Banshee. The mana spread is poor (only covers 4 colors) but it’s usually possible to keep it going for a long time. Good for map farming, too.
That is my standard team these days (my order: Flame, Alchemist, Banshee, Valkyrie), generates more maps than I could ever need and a hefty amount of souls. Only downside is it is slow. I’d taken to using 3x Skel + Orb for the faster kills, but today rendered that unreliable.
Great, happy for you. The alchemist I’m sure really increases the effectiveness. Let me know if you want some more help; I’m just a bit ahead on the curve from you so I know what you’re going through.
What Sunweaver does is it really helps me come back from a board I lose control of. The +6 mana helps get the roll going again, the +7 armour/health and attack really help with the skulls against some of the really tough teams (goblins/dragons).