Lahair / Hogle / Lancelot / Hector / Achilles Quests (7 star powerhouses)

I'm kind of lumping them all together because when I was going for these cards, I used the same strategy.  For all cases except Achilles, you have no heart diamonds, meaning to heal, you'd need either auto-healing or a skill to make hearts on the board.  I went with the former.

General setup:
Leader: 50% damage reduction of the element featured
Ally: Strong healer
(Of course you can swap the roles if you have the cards/allies)
The rest: whatever really.  RCR is irrelevant since auto-healing will cover health, not heart diamonds.  If going for Achilles, you can try to balance having some RCR.  You can have an Erinyes to buy time for auto-healing if it cannot cover the damage from attacks.  Cards with iron wall can help too.  Remember to consider what element does more/less damage if deciding to go with HP over damage.

Quick Notes:
- It's purely mono-elemental.
- Nothing should hit really hard with this setup.  This also means skills like surviving a strike with more than 40%/70% HP won't help.
- For level I and II you don't necessarily need 50% damage reduction.  30% should do fine.
- Level III damage is not tremendous, but those reduction cards do make a big difference
- Achilles' quest prevents dark diamonds, not heart diamonds, from appearing.  So this strat would still feature Dark Witch+++, though she can't do much except act as defense and occasionally throw her attack skill.  However this means you can heal with heart diamonds.
- If you are just getting your feet wet in this series (as in, you're new to this) and don't have access to strong damage reduction and healing cards, you can start with a regular Jotunn and use her skill to generate hearts when needed.  To prep for that, build combos not invovling fire diamonds, then use her skill to get lots of hearts from it.
- The cards themselves are powerful, but a little expensive on the cost (40).  Their stats at base don't seem too powerful for a 7 star, but they make amazing leader cards.  Their skill is a 500% damage attack, but on a short cooldown.  If you can stack another damage buff, it can get a little out of hand.  Plus you can increase the skill level to reduce the turns to only 5!