It was not an easy go. I did lvl 400 using Harpy Mage, Harpy, Bladewing, Xochi, and thought that would be the ticket for 500. But I kept running in to the problem after a few weeks of trying, especially in the boss room, of being unable to summon a new Harpy Mage when I most needed it. So instead I dumped Bladewing for a second Xochi, and I placed the second Xochi in the second slot because my entire strategy revolved around summoning a storm with Mage, and using Harpy to mana drain the opposing team’s board control. If Harpy dies, you are pretty much done, so I could not risk having her in the second slot.
First fight: As noted above, my strategy was to focus on keeping my mana topped off for my troops, and only casting Mage when one of the opponent’s Mages had full mana, or to get out of bad boards (e.g., a skull match, giving up a 4-match on the next turn, or giving up a potential board reset). I would ignore casts by their Harpy or Bladewing, and cast Harpy every time she was full, to try and build up a reserve of life for unlucky skulls.
After I cast Harpy Mage, I would use Harpy to drain the opponent’s team of mana, thereby hopefully denying their Mage the chance to fill their team’s mana. If they refilled a Mage or Bladewing after my first Harpy cast, I would cast Xochi or Mage again to try and refill my Harpy to drain them again. You need to control their Mages because ultimately you need to control Bladewing - I had a hard time getting a run started at first because Bladewing one-shot my Harpy a few times.
You need to be very tactical in the first (and boss) fight with your Xochi casts. Do not kill any of the back three enemy troops with spell damage unless you are confident you can kill off the rest of team in short order - they will resummon a Harpy and prolong the fight. It’s kind of interesting in this delve, how you have to play conservative for the most part, but when you have them down, you need to take more risks by exploding the board with Mage and Xochi to keep from giving the other team a turn (and therefore a chance to summon another Harpy). Do not hold back when you have their last troop low - it’s better to lose your Mage to skulls than give them a chance to summon a Harpy (which can quickly lead to two Harpies, etc.).
Middle rooms: I usually would take the rare room paths for most runs. You are looking for Coral Golem or Shadowblade for the higher rarity rooms - these are doable without a ton of trouble. Ice Wraith, Dire Wolf, and Morterra are doable in a pinch as well, if you get unlucky rolls on the rare room paths.
For the rare rooms, I would avoid: Bat Swarm (your team is too slow to keep up with their transformations), Spider Queen (you can’t drain Dark Priestess), Golem (Dark Monolith hits too hard), and Pirate (your team is too slow to keep their attack from scaling on you). (On my winning run, I got Flame Troll and Leprechaun.)
Otherwise, my strategy was similar to the first room - use Harpy to mana drain whenever the enemy’s board control was about to cast, to slow down the opponents’ casting, use Harpy otherwise to build up your life, sneak in Xochi casts when you can’t match greens/browns for Harpy.
Boss Room: This worked out very strangely for me. I executed my standard strategy well for some time, which really limited their Harpy’s ability to buff their team’s life, and killed Harpy Mage and did good damage to their Xochi. The problem is that it’s really hard for Harpy’s healing to keep pace with Xochi’s damage. I was in trouble after a Xochi cast I couldn’t stop got through, and killed my Mage and my Harpy at the same time.
Whenever you lose Harpy in the boss fight, there’s only one thing to do: yolo!! I needed several things to go right at this point, and they did: I got a 4-match to summon another Mage during a dust storm. This allowed me to cast a Xochi, and get mana quickly for my Mage. Before long, I had summoned two mages, and was able to fire off multiple exchanges of Xochi-Mage.
I lost my front Xochi to a Bladewing cast (not due to lethal damage, she had taken a skull hit earlier), but not before I had taken out the enemy Xochi. I I was able to summon a third Mage, and from there the loops just kept coming. I dropped the turn a few times, but did not get burned by a summoned Harpy, and did not give too many bad boards. It was nerve-racking at the end, but a Xochi cast, with a skull match after the extra turn, were enough to take down the last two troops.
It only took a couple of days with my new team to pull through, and I couldn’t honestly say if it was strategy or luck that won the day. I do know that it felt amazing to get through this RNG fiesta with a win though. Good luck, and hope this tale can be of use to you both.