This achievement is actually glitched, Would like to say this in a solution. Basically, the combination is the same
A, LB, RB, RT, RS, B, A, LB, RB, RS
However to prevent a timer from making u rush this part ( this works really well on hard) done it 3 times now and works each time. Just before you get to the bomb bit, you've gone down the stairs cleared out the first lot of enemies then you pass through the slalom area. Dont go through, hang back, shoot the guys you can see, try to clear out as many as you can, then go forward. This allows both you and your partner to take up the positions for covering the stairs and disarming the bomb easier. Just to confirm aswell,the 20 seconds is from when you press B, not when the timer is supposed to have gone off. IF this glitch is done correctly there should be no 60 second timer.
Any negative votes, please mention why, I'm simply just mentioning a glitch in the game to help people, not compelling you to do it