Captain_K wrote:You do not need to see something to hit it... if its in melee combat the penalty is steep but not impossible. Second, Is the 23 AC the result of not seeing? Then you must hit that AC without more no see penalties... if you see the AC become typical like 15 or so. I think "invisible" opponent is like +10 to AC or at least +6... so take that away from the 23 and you can hit it... so hard to hit unless you get lucky and see it, then easy to hit... seems fair, you only have to hit it once, do any damage and it will cut bait and flee.. nothing kills itself for a payment... it will believe it is safe until it take damage... area affect nails it pretty well.
Plus, you as the DM could have a little fun with it - remember it is fay and chaotic.
Have it attack from surprise, the PCs being flat footed, then once the PCs know they are targets, have it laugh loudly form one spot, then quietly invisibility move to another, and then show itself, attack, and disappear again. have it move around for a bit and not attack for a few rounds, then attack again. Play up the cat and mouse for a while.
Also, after a bit give the players perception/wisdom (how ever you handle noticing things) check after the attack or during the round or 2 of cat and mouse movement. Invisible is not unnoticeable ... a foot print here, a scrape of a weapon on a tree there, a tug of a cloak on a bush to the right. Again, build the suspense. However, don't make it impossible, give them a chance to be a little focused and not just wonder around swinging at random and hoping to hit that 23 AC with only luck.
Plus, like Capt K said, area effects ... you don't have to see something to fire ball it ! But also, a good hand full of ashes from the evening fire - could cover the little bugger and defeat the invisibility power, could blind the critter and even the odds, or at least cover an area and deny that area from the fey and not let it run through it without leaving tracks in the ash and foot prints after he leaves it - is a good area effect that the party's fighter or rogue could easily use.
Then, it isn't a fight to the death ... the quickling could kill the party with a thousand little cuts, but at 1 HD, a good hit from the party could put it on deaths door. So, once there have been a few close calls or a hit on the fey assassin it will exfil and may or may not come back for round 2 or 3.
I remember back in the day, good lord now that I think of it, WAY BACK when I was in high school, a similar situation with the party I was playing with and a twisted DM who loved a mean little vindictive leprechaun ... We quickly learned to HATE that thing ! Eventually even the party thief would just leave his best gem or a hand full of gold out when we were in an area the leprechaun was know or expected to be in, and let it take its 'green tax' without emptying all our packs, cutting our bag bottoms, untying our horses etc.