Hiya!
I've known about this since Cataclysm and been doing it since then.
First off, to all of you who don't know what instance Sunwell Plateau is, it is a raid that was introduced back in The Burning Crusade, and the final boss dropping a legendary bow Thori'dal, the Stars' Fury - Item - World of Warcraft
You will need two people on the first boss in order to be able to kill him, but since he's not required to kill to be able to defeat the final boss, then why bother? With this you can run the instance whenever you want without having to look for a group or a friend to help you kill the first boss, and this will bring you more chances of getting the sought after legendary bow by soloing the rest of the instance. The only boss that could be a pain is the second boss Brutallus, it requires a bit high DPS or you being able to survive and taking huge hits on you.
So, as the title says, Kalecgos is easily skippable by simply running around him, either to the left or right hugging the bushes:
Have fun getting your legendary!