Eventually you’ll reach a point where buying up every kingdom and playing through them might not be so possible. So I’ve recently constructed a team that’s both easily obtainable and effective for exp grinding.
A few things you’ll need first, you’ll need to have unlocked both Whitehelm and Grosh-Nak
Play through the Grosh-Nak campaign far enough to have unlocked the 3rd Challenge (Summon the Horde).
You gain exp for every troop you defeat, including summoned ones. This is why this challenge is perfect!
Here’s the team: (Flag +2 Yellow, aka; Whitehelm)
Goblin Shaman (Lv 10 at least)
Sparkgrinder/Sunweaver (Lv 10 at least) (Sparkgrinder’s faster to obtain, but Sunweaver’s better.)
Wight (Lv 10 at least)
Paladin (Lv 15)
First plow through the cyclops to stop their spells crushing your team. But keep the Summoner’s alive as long as possible.
Use Sparkgrinder/Sunweaver to power up Paladin to about 15+ Armor (power Shaman when he gets weak afterwards), use the Gob Shaman to power Paladin’s spell which should only ever be focused on Cyclops and Ghouls. Wight is here to fasten the Summoner’s spell, as you should avoid grabbing purple and red gems even if they’re 4-5+ matches.
If possible as suggested, doing this on multiple devices should excel the pace. Just avoid ending rounds at the same time, it can cause errors to occur occasionally.
Can’t say if it’s the fastest way, but it works so try it out when possible. (You’ll also always make more than you spend on the challenge if played right. So no down time!)