To get this achievement, host a game and make the level Manor House with five targets. The bomb spawns upstairs on a bed in the manor house. Set the bomb on a generator and wait 15 seconds before it explodes - that counts as one target. Go back to where the bomb spawned and grab it and do it again. There is only one bomb per level.
While boosting achievements, set the targets to three targets and have the time at five minutes. Start the game and have someone from both teams get two targets each. Now the person getting the secret achievement "Last Second Hero" get the bomb and wait by your final generator. When the timer hits 17-18 seconds run up to the generator and get the bomb on it. At about 3 seconds left, the game will end and the person who is going for the secret achievement should get it.
Note: You can not play 1 vs 1 ranked matches. Minimum is 3 vs 3!