Start your attacks with the special ones that have a longer cooldown. Also use heroes whose special skills are best used early.
Coins
Play against other players in the arena to earn coins if you have a large amount of battle-ready heroes to choose from. Sell unneeded equipment for coins, but only after having a large number of heroes that are already equipped.
Experience
Heroes gain experience during battles based on their activity. To level a specific hero faster, us it more often than the others. A more riskier strategy is to keep most of the other heroes back and charge with the hero to be leveled. Pair that hero with the guard and healer. Keep attacking with that hero and avoid dying.
Silver Heroes
The Silver Hero costs 6,000 coins but is worth the investment.
Skills
Invest the coins you earn into teaching your heroes new skills or improve existing ones. Most teachable skills are passive and are always active. Once learned, that hero will always get that bonus. Each hero has their own set of skills. Focus on those that will help their role. Purchase all the possible skills first because the bonus is generally greater than what an upgrade offers.
Troop placement
Use the terrain for your advantage: Attack from higher ground to get bonus damage with your archers. Move slowly to remain in the protective range of the guard and healer. Eliminate enemies one at a time to avoid taking a lot of damage.