Play it till your eyes bleed, sleep then play it some more.
Thats what i did over christmas and now I feel im pretty good. Upclose, cant beat the shotgun. Makes the chainsawer's think twice when you put an active reload in their face....
Practice with the longshot. Play decent players rather than people who try and chainsaw you all the time and use active reload are my tips on improving quickly.
Top tip tho - Longshot no-scopes are the way forward, either headshot (when you get good) or body shot to take them down and finish with an execution.
Stay in teams of two at all times is another tip, specially when playing with decent opposition. Someone with a mid-long range weapon and another with the shotgun making sure you go round corners with an active reload