Only suggestion I give at the moment is what many others have been telling me. Don't 50 or 60 any characters unless you intend to take them to the end game. Use your resources wisely unlike I did. Happy hunting!

I'm by no means an expert but first thing I'd suggest is get Sethallia to 60 asap - her heals, buff/debuff and TM control are too good not to have her maxed.

Midgame is not really exactly defined. But I don't think you are anywhere close to what I would call midgame.

For Iron Twins I can just give a general advice: Bring a reviver and try to let one or two champs die before the robot falls below 40% hp and gets his killer moves enabled. If a champ early on gets decrease def and falls to low hp, don't heal. Let him die and revive. All debuffs expire at death, revived champs return to the fight without ironbrand counters.

You have a key champ for the Nether Spider, Reliquary Tender. Her remove debuffs is on a very fast cooldown and she gives a heal buff when removing the poison. 

You have some more great champs that deserve to be lvl 60. Not for the particular stages you have problems with, but I guess you are not at stage 25 at the old dungeons yet. Venomage, Sethalia, Cardinal, Armiger can go to lvl 60 for sure.

tl, dr: 12.3 brutal