TL;DR Use Ragnaros, Millhouse, and Cariel. Use "DIE, INSECTS!", "Arcane Explosion", and "Taunt". Watch your enemies melt.
Shockingly I haven't seen a single person online mention this combo yet, so I figured I'd just let the community know that Ragnaros, Millhouse Manastorm, and Cariel Roame (along with whatever three characters you want to level up) seems to be BY FAR the quickest way to grind experience as it generally wipes out all enemies (even Level 30 ones) on Turn 1 as long as they have no more than 37 Health (or 49 if they're Protectors or 62 if they're Fighters). In addition, picking up treasures (or having enemies take damage when they smack into Cariel) can make the kill ceiling even higher. And if the enemy does somehow survive the first turn, you'll have no issue taking them out on a later turn instead.
Ragnaros' "DIE, INSECTS!" is what you're using to wipe the entire enemy board. Millhouse is just there for his "Arcane Explosion" to help make sure all the enemies' health is low enough for Ragnaros' ability to keep on reactivating. And Cariel is just there for her "Taunt" to help make sure Ragnaros survives. Those are the three abilities you want to focus on upgrading. If you have extra coins put them into upgrading the equipment "Blazing Rune", "Arcane Powder", and "Tome Of Light" to further strengthen the combo. If you have even more extra coins feel free to upgrade those Mercenaries' other abilities as they can sometimes be useful in certain niche scenarios (primarily against the final boss of each run). Also as a side note Millhouse and Cariel are technically replaceable if for some reason you really need to, but I highly recommend against it especially if you're fighting high level enemies.
Of course most people won't have maxed out abilities and equipment so just for an idea of how it scales; the threshold for killing everything on Turn 1 is 37 Health for Neutrals / Casters, 49 Health for Protectors, and 62 Health for Fighters (let's call that 37/49/62). Without equipment it's 30/38/52. At only Level 4 abilities it's 27/34/47. At Level 3 it's 24/30/42. At Level 2 it's 16/20/28. At Level 1 it's 8/10/14. I'm not gonna bother going into equipment levels or if one character has a higher ability level than the other. Upgrading both abilities to Level 3 is a huge leap and will carry you quite far, after that any upgrades are marginal although still definitely helpful nonetheless. Also keep in mind these calculations are just for if you want to quickly wipe every enemy team you run into on turn one. Also if you have treasures, an enemy heals, or an enemy takes damage smacking into Cariel that would effect the calculation. Regardless, hopefully this all ways helpful. Good luck grinding!