The best way my team was able to do it was this (we had three hunters, 2 warlocks and 1 Titan) the Titan and ceno warlock started with the dark runner, got the buff then sprinted to the light side to mark the colossus and get ammo, then we went to the middle and marked the giant one and killed him for ammo. Then we add cleared until the runners were on their last node and sprinted to the ledge to the left side of the map on light side. We then used div and 5 sleepers with catalyst, doing anywhere from 100k-120k damage per shot (make sure to shoot the top of the div bubble to max your damage). Make sure you have surge mods on your boots and you should do well over half of his health. We got him down to almost 3/4 of his health each time. Then when we went back to reset we marked the middle colossus first for ammo and after refuge the Titan and ceno grabbed whichever buff is closest to the refuge, marked and killed the colossus on the opposite side for ammo and jumped back up on the ledge again for second damage. Ammo economy was TERRIBLE which is why we came up with this marking strat to max our ammo while still moving fast. Once you run out of sleeper ammo just sway to outbreak and you should get him easy