Have lots of detect illusion potions (the ones sold in NR shops are level 70, you'll have to get the one in Rahdiyr equipment box in the dungeons). Also get some healing objects and octarine wands to damage the beast with as it has a lot of HP. Head to Faerie Plane via High Priest of Wroth (password is Hala Katalla Hum), jasper gemstones or worldgates. Go to Cavhfail and obtain a Cavhfail Keep Password.
Directions from sentry post: w,n,2w,nw,ne,u,e,n,2e,n,d,e,n,2w. search for a hidden south, 2 s, have detect illusion on and go south into LPK.