Regarding the Explosive Finish achievement when making your way down to the mainframe, try and get a weapon like the Ak-47 to replace the Recurve Bow. When on the mainframe attach the hook. Whilst you are going up, keep on constantly looking around for the red barrels as you only get a few seconds on each floor. I would recommend doing this on Recruit.
If you are still struggling, feel free to use this video as a reference.
Hope this helped.