1.) Waste a clip of lancer bullets
2.) Keep pressing B so it keeps trying to reload
3.) Go to a wall that you can take cover on (wall not barrier)
4.) Go forward off the wall and press B once (if done correctly it should make your lancer not reload and your clip will stay empty without pressing B)
5.) Have someone ragdoll you(not sure if cringe works???)
6.) Now take cover on a barrier and hold B (you cannot see yourself chainsawing, but anyone else can) You will be behind cover and chainswing at the same time.
7.) Mantle the cover and when you are just about on top of the barrier Hold B+A+Down on the analog. (you should be on top of the barrier)