Invite your friends to balance your team. Soliel is weak against arrows, bring at least one good archer.
Leading to the fight there are two paths to choose from - left one is longer but awards a ring (+3 def +10% fire resist).
The battle itself has six waves (3,3,4,4,5,6 - mostly skeletons and demons), so be prepared for a longer encounter and have health/mana potions ready to keep the party alive for the last wave.
A demon in the wave prior to Soliel can cast paralyze, be careful to avoid going into the last wave while paralyzed.
Soliel herself will be encountered during the last wave. You'll need to clear the entire front row to be able to attack her in melee. She has 3 spells at her disposal: Shade Charge (deals 1,5 heart dmg and lowers hit chance for 4 turns), Flaming Skull (3 hearts fire nuke) and Summon (zombies).
Soliel can be disabled by many means - frost bombs, frost bolt, paralyze, confuse, throw voice, etc. Having her minions target her with throw voice can be very helpful, she'll go down faster and you won't be damaged by the minions during that time. Use a mage to cast mass paralyze for crowd control, a rogue to put a mass volley in the air, and a warrior to drive through the front line with overrun. Dropping a time bomb on Soliel may also prove helpful. A single poison bomb will damage her by a half heart at a time. Seeing as there will be 9 (3 party + 6 enemies) characters on the board she'll get at most two turns in that time.
The bounty for killing Soliel is 800 and 100 XP.