After complete some levels, you will unlock the option to play in Elite mode. In this mode, the enemies are stronger and treasure is better, including hero and angel soulstones.
Heroes
Obtain the following soulstones to unlock the following heroes. The soulstones are only found in Elite mode. You must complete a stage in Normal mode before you can play the Elite mode version. Other heroes are only available through card draws or the Arena shop.
Warriors
Berserker Claw: Flame Wrath, Moon Howl, Terror Cove.
Blood Baroness: Card draws.
Dark Paladin: Card draws.
Earthshaker: Dark Forest, Boreal Forest, Death Lair.
Headless Horesman: Card draws.
Hecate: Nightwatch, Deathknell.
Hell Hound: Card draws.
King o' Bones: Watchtower, Infinity Road, Fire Forest.
Shadowraith: Shadows, Wraith Attack, Sardin Cliffs.
Valkyrie: Fierce Path, Stone Canyon, Mist Plateau.
Zweihander: Purchase from Arena shop.
Mages
Arcane Spirit: Thunderclap, Elementals, Blaze Altar.
Astral Child: Card draws.
Blazing Soul: Meteorolite, Fireg Laputa.
Blue Witch: World Forest, Doomstone, Wood Upland.
Byron the Black: The Abyss, Bloodmaul, Thorns Wall.
Djinni: Purchase from Arena shop.
Exorcist: Cold Blade, Ash Cave, Mist Town.
Guardian: Storms Altar, Dark Outpost, Mystic Fort.
Light Envoy: Card draws.
Lunar Priestess: Lok Bridge, Despair Cliffs.
Mermaid: The Barrens, Disaster Root, Fort Hall.
Nereida: Inverted Cliff, Arctic Furnace, Endless Sea.
Soul Slinger: Card draws.
Summoner: Foultusk Lair, Hammer Fold, Wind Forest.
Archers
Amazon Hunter: Assassinate, Fire Cage, Torrent Camp.
Astral Hunter: Accursed, Doom Citadel.
Demon Hunter: The Nightfall, Demon Cave, Arctic Domain.
Grimbow: Purchase from Arena shop.
Night Sentinel: Purchase from Arena shop.
Nightblade: Dragon Youth, Mist Valley, Arctic Beacon.
Snow Queen: Peace Zone, Titanhammer.
Heroes
Before you stop actively playing, have some heroes in your training slot. You add to the three slots you start with by spending 500 diamonds. When heroes or angels are placed in those slots, they will earn experience over time, whether you are actively playing or not.
Promotion
Each hero has six equipment slots. Some of them can be filled with crafted equipment. To do this, find their base pieces. Tap on an equipment slot to view where you can find its base piece. After you have all six equipment slots filled on a single hero, you can fuse the equipment with the hero to promote them. Promotion gives that hero a boost to stats and will unlock new abilities. Use the Blitz option to complete levels quickly and farm for equipment.
Stars
Do not allow your heroes to die. If you complete a level without letting any of your heroes die, you will earn three stars for that level. The amount of stars you earn has a minor effect on the gold you receive. However, earning three stars unlocks Blitz mode for that level. Blitz mode allows you to complete a level and get all possible drops without actually playing. This saves time later when you are looking for specific items. If you cannot get all three stars on your first attempt of a level, return later when your heroes are stronger.
Upgrades
Because you will be using the starting angel, Vienna, for some time, upgrade her as much as possible. Improve her two active abilities and her only passive ability, "Divine Contract". This ability conveys an attack boost to all your heroes at the start of a level. The prices for upgrading Vienna are very low.