I play PS4 so this might not relate to PC controls.
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Stay a medium distance away from your tanks and you can heal and do damage at the same time by pressing L2+R2 at same time.
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Do not engage anyone who is head peeking. The upwards recoil will insure that you never land a shot.
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Vary the height of your jumps to throw snipers off as best you can.
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When jumped use the field to save yourself make sure too look down before you hit R2 or you will toss it too far.
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Use the super jump to toss the field a very long distance.
Basically he is a mix of junkrat and soilder with a more accurate weapon than soilder imo.
I can actually hit mercy now.