Solitary Colossus is large pillar standing near south entrance to Ruins of Morah.
While the Solitary Colossus may appear unreachable, it does not need to be attacked with Junundu Siege - just approach the side of the rock platform on foot and use something like Immolate on it. As it is only level 1 (level 22 in hard mode), it is pretty easy to kill. Once it is dead, you will be able to walk onto the platform - and onto a new rock bridge which takes you across the chasm to claim your reward. Counts as a Monolith for the "Monolith Hunt" Bounty.