My soldier had an easier time witht he Collosus , than any other guy...wlel and my Infiltrator those two kicked ass.
With my Vanguard, I took it a bit slower, charged straight right to the passage way on top, left Garrus and Zaeed Sniping back at kal regar, and they lured most of the guys to them.
I killed a few guys on the passage way to the right, moved up without charging, and as soon as the colossus electric bolt hit my cover, I would charge him.
I charged him from a small balcony crossing the open passage way. Hard to explain.
If you go through the passage way on the right. you'll pass a down side ramp. 5-10 feet from that ramp is a wall of metal sheet, there is a break in the sheets that you can shoot the colossus from, I charged from there, called my guys quickly so most of the geth went after them, and by the time they arrived I had killed the colossus with more charges and my shottie