Wanna get some rare heroes? Your best bet is to go for the gold chest, which is free once every 46 hours, and costs 288 gems apiece at any other time. Rare heroes often appear here – not rare hero soul stones, but actual rare heroes. Usually, though, it will be rare equipment that appears here – stuff with the green and blue outlines.
The other way to get heroes common or rare, of course, is the soul stone summoning, so check the locations of all of the soul stones by looking at the profiles of any hero that has not been caught. All of the locations will be in elite dungeons. You can play all elite dungeons three times per day apiece, so play EVERY dungeon three times a day for the best chances at getting enough soul stones for some summons.
You’ll need a ton of gold coins to keep up with upgrading your heroes’ skills, and one easy way to load up on the gold coins is simply to sell all of the mithril that you get. Mithril is useful for literally nothing, but each one you sell will earn you 5,000 coins, making it pointless not to sell them.
One thing that’s less obvious, though, is to sell your EXP potions and vials, because potions are worth 400 per, and each vial is worth 1,200. You always end up with WAY more EXP potions and vials than you really need because of how low the maximum hero level generally is – it usually matches the max player level, give or take one level or two.
The key to beating tough opponents (tough meaning having slightly more power than you) in the Arena is to have your skills upgraded sky high. Skills are not counted against your overall power rating, but skill upgrades pack a HUGE punch and skills can be upgraded almost endlessly. All that you have to do is have your characters be strong enough to stay alive in battle up until the skills can be activated.
Wanna make it really easy to get coins and upgrade skills? All that you have to do is go to any elite stage, as they tend to have high concentrations of Mithril and EXP stuff, farm the hell out of it, then sell all of the mithril that you earn once you run out of stamina or elite turns.