What weaps do you have? Magras can be a bit tough with the lower tier weapons but perhaps you should try the grav cannon and take your time and line up a one shot on him. Now I lay waste to him with the spitfire and the divider but i do recall that I killed him in my first or second playthrough with the grav cannon or the maelstrom bomb with a well placed hit.
Another good way to to get a high lockon count of missles on him but don't fire. Then keep shooting at him with your nose turret of choice until he does an evasion move. After it completes let the missles fly because magras won't do two evasions in quick succession. This might help you a bit hopefully
TeamFortressTitans/TollFreeT-bagging
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