First of all. There is an error in quest description - do not look for this boss in Yon-Chall Forest. He is in Lost City on a 3-rd level.

You need to know only three things about of Crystal Spider
A) He is very good at distance, but sucks in melee.
B) His main weapon is Impolson, so he do a huge amount of Prime damage (and also lot of Dark)
C) This guy is extremely magic resistant

So we need good Prime resistance, first of all. This can be done by two ways: by using a scroll or by leaving a beacon near boss, then finding an appropriate statue and jumping in. Arcane Ward will be also quite usefull.
Also we need two or more melee fighters with Warfare Expert skill. Now we ready to kick his ass.

Our first task is to break the distance. Buff yourself as much as you can and move forward. After short animation you will find yourself in narrow doorway. Move forward and take first magic strike. On a next turn move left. This guy is little stupid - he can't move on square with chest so he will not shoot anymore. Move closer, stay on a chest and start fight. Do not try use attacking magic or debuff him - he will resist most of your attempts. Even if you manage to hit him, damage will be minor. So your casters have to focus on buffing and healing party. Use advanced attacks like a Unstoppable Strike because he often block simple attacks. Time to time he will use Crystal Cage or simply shift away. Then you have to move closer again and resist to his magic shots, so look after your Prime resistance carefully. Keep feeding him with cold steel and eventually he wil die.

Good luck, Raiders.

P.S. Do not forget to take Fire Shard from small chest.
P.P.S.from Russia with love.