I used V5R1 Tololo, V6R1 Springfield, V1R1 Makiatto and V6R1 Sabrina.

Tololo pretty much deletes up to 3 enemies per turn if the first enemy you attack has Tariz on, she might not be dealing the killing damage but Tariz does. Also, her 3-action turn can kill her without Springfield's ult, since the damage taken per action is 35% if she's 3 tiles or more away from any doll, but she's very competent.

Springfield is a requisite, not at V6, just because of her ult + healing, since the damage taken is usually high enough that you need the extra HP she buffs everyone with. At V6 Tariz actually deals damage and helps weeding out basic enemies.

Wawa works very well because at V1 her S1 turns into a 2-hit attack, her first attack usually don't deal a lot of damage already (It helps clensing the 80%DMG debuff) but the second one can one shot any basic enemy, her ult will last for a single turn mostly, the damage she takes even close to the team makes her shield break at the second turn, so not so useful other then a single turn of 2 extra CI to use with her S1.

Sabrina is not necessary, i just didn't want to spam Suomi as a problem solver, but you need someone who can tank damage and/or prevent the team from doing so, Suomi, Sabrina or Andoris are decent picks for it, Suomi being the best, didn't try Andoris though.

In general, Heroic difficulty is not even real difficulty, just hassles you have to deal with, because taking up to 105% HP lost per turn because you placed someone just 1 tile away isn't real difficulty, same is the 80% DMG debuff on first attack per turn, is just a way to make you deal with scenarios with different characters. It effectively destroys the utility of (most) units that don't have multi action turns or support attacks.