I don't know why i had so much trouble with him, but i did.
Cabalion is the steel fighting type legendary pokemon found in Mistralton cave. to get him you need-
-a pokemon that knows surf.

Cobalion is lvl 42 when you find it, its attacks include iron head, sacred sword, etc.
To get to him, go towards the beginning of chargestone cave. Look for the house near the front of it. Around that area is an old man, who will tell you that across the river there is a cave where a legendary pokemon lives. If you use surf you'll find that hes right and arrive at mistralton cave. Not long after looking you should find him. After beating/cathing him, the old man will tell you a tale about three fighting legendaries and that they are all waiting for you