Killing the Guardian **Spoiler**
Your party "should" be capable of handling the Guardian. My first party had no problem and they were lower level than yours (not much). This was my first party and run through of the game. I would have to pull up the autosave to check them but I "believe" my party was as follows:
Paladin 13th (Assimar)
Ranger 13th (Assimar, don't know why I did that)
Fighter 10th (Sverfneblin)
Cleric 14th (human and of Lathander, might make a difference)
Fighter/ Mage 5th/9th (elf)
Barbarian/Rogue 6th/7th (Tiefling)
First, Don't use Arrows of Piercing! They aren't what they say they are. Yes they do cause extra damage if no save, BUT, the description says they have +4 enchantment (IIRC). My experience in the game says they do NOT. They act as standard arrows with a special ability of causing extra damage. But they do not act as if they are enchanted, especially as +4. I tried some on Iron Golems and other creatures requiring enchanted weapons (nothing!!, did not work). Instead use "Sureshot" (which are enchanted as +5 although do not give +5 to hit) or +2 or +3 arrows and bolts. The bolts of sparking "I think" also worked on it.
I suspected that the Guardian was some type of Dragon since we were dealing with Yuan-Ti.
My Paladin was immune to the fear as was my Ranger who was wearing the spiked dog collar from early on in the game (great little item for as early as you get it). So I set my Paladin & Ranger on the steps, weapons in hand. I "think" the Paladin had +3 sword and the Ranger had the "Sure strike" short sword, not sure though. The others stood back in the grassy area with a variety of missle weapons at the ready. Summoned the Guardian with my Paladin and quickly moved back to the steps to block it off with the Ranger.
**Spoiler**
Clerics of Lathander get one great kick butt domain spell (as a 6th level spell) much earlier than other clerics do as a regular cleric spell, Firestorm. It seems to bypass magic resistence, (because it is a divine spell vice arcane perhaps?). Once it appeared my cleric cast Firestorm and my mage cast Lower Resistence at the same time. Lower Resistence has shorted casting time and goes off first. The Firestorm hurt it pretty good. The missle troops started firing away. After the second round of the Firestorm, it took a step backwards. I then sent my Paladin in on one side and my Ranger in from the other. They seemed to box it in and it never moved from that position the rest of the fight. It kept turning back and forth to face one or the other but never moved away. My fighter/mage using a bow moved himself closer to get a better shot and ended up running in panic, but other than that others were far enough back that they could shoot at it and not get panicked. A couple of rounds and down it went, no deaths. So I am thinking that if you can pin it in one spot it might not have the freedom of movement or the desire to teleport around. I may have been lucky, but it worked
![Stick Out Tongue :p](./images/smilies/)