Ignore equipment dungeons until you have full team lvl 50 heroes. Only do minothaur for tier 1 and 2 masteries. Only do the minimum required by missions.
Always concentrate on champ leveling/ranking in campaign, especially when you have EXP booster up.
Concentrate on maxing one hero first, equip best items you got (preferably some lifesteal set) level them to 12. Use exp potions if needed. Your goal is make him be able to solo certain campaign mission. The farther - the better. Try to pick this first lvl 50 (or later - 60) champ smart, think of it as long time investment. It doesn't have to be the starting champ. Kael is good against CB. Athel is good against CB and Fire Knight. However, they will need books to max skills, while Executioner can be very effective in campaign or arena only but is much easier to max skill.
Once you get one efficient farmer, it will be much easier to quickly level up new champs in case if you're lucky to pop something decent from shards. If you don't pop anything cool from shards - keep leveling food (useless rank 3/4/5 champs you'll sacrifice to rank up your good champs) and throwing it in your stash.
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Only then, when you don't have EXP boost, don't have any doable mission, don't have any other priority objective - only then you can waste energy on equipment dungeons :)
