The Obelisk puzzle is designed for two characters. One to lure the trimorph away, and the other to put the chorus in the obelisk. However this can be done with one character with ease. Take the character with the chorus to the obelisk. Run around the right side of the pillar (the character's left) and make a wide circle around the trimorph that is now chasing you. Make a circle to the pillar where the trimorph was standing and insert the chorus. DO NOT BRUSH UP DIRECTLY AGAINST THE PILLAR, because you will get stuck and die, instead have some distance between you and the pillar when inserting the chorus. Then immediatly break into a run and return to the spot that you started from.