Two possibilities, if Solstice uses only known spells:
Round 1. Fooruman raises Araches/Leetsan, defends. Faw goes NW. Grandma W. Sam defends. Araches/Leetsan defends.
Round 2. (1) Solstice can use Kudzu to cause 7 damage to Araches/Leetsan, leaving them with 3 HP. That leaves the hydra to attack Bob/Faw or Fooruman. Faw casts heal on Araches/Leetsan and attacks ICE HYDRA, killing it. Everyone else defends. (2) Solstice can use Kudzu and, with ICE HYDRA's attack, kill Fooruman. No revive for Araches/Leetsan, one heal lost, no ramming speed. Faw casts no spells, attacks ICE HYDRA, killing it. Grandma attacks Solstice.
Round 3. (1) Solstice attacks Faw, causing 14 damage, leaving 10. Fooruman can proceed with ramming Solstice, and Grandma attacks Solstice. Araches/Leetsan can use Mythslayer and cause 2 damage to Solstice. (2) Slugging match between Bob/Faw, Grandma and Solstice. Solstice could use summons to give him an advantage, while we have nothing.

Probable outcome: Round 1, round 2 (2), round 3 (3).

Round 1. (1) Fooruman goes N. Shuriken casts Haste on Faw/Bob, and either attacks the box or defends. Faw/Bob goes W, NW, attacks ICE HYDRA, killing it. Grandma goes W. Sam goes 2xW. This seals Solstice in and prevents him from teleporting out. (2) Exactly the same as (1) except Faw/Bob goes 2xW and attacks ICE HYDRA from there.
Round 2. (1) Solstice uses Kudzu and attacks Fooruman. Fooruman uses Raise on Araches/Leetsan and defends. Bob/Faw and Grandma attack Solstice. Araches/Leetsan defends. (2) Solstice has only one chance of escaping and teleports somewhere else. Faw goes N, and blocks off Solstice from Araches/Leetsan. Fooruman raises Araches/Leetsan and defends.
Round 3. (1) Solstice attacks Fooruman, who attacks back. Bob/Faw attacks Solstice. Grandma attacks Solstice. Araches/Leetsan uses Summon mythslayer next to Solstice and defends. Mythslayer attacks Solstice. (2) Araches/Leetsan uses Summon mythslayer S of Bob/Faw, and the mythslayer attacks Solstice. Bob/Faw defends. Sam defends. Fooruman defends.

In the next round, regardless of (1) or (2), Mythslayer can reduce Solstice's HP by a third and Araches/Leetsan can use lightning on him. If Solstice does have summons, mythslayer will probably die, so we may have to change him with something else, except we know nothing about the enemy summonables.

Probable outcome: Round 1 (1), round 2 (1) and round 3 (1).
I will have missed some things out, but nothing too important, so you can see that the second possibility is much more beneficial to us.