Hi Enperry,
Some thoughts...
Turf War's last 30-45 seconds matter most, but it's really about map control. If you hold mid, you win. There's some strategy in terms of holding choke points on maps, watching flanks, and trying to fragment the opponent's efforts, but nothing too elaborate. If you can communicate with your team via voice chat, you can easily coordinate a push onto the opponent's side and hold it. With randoms in Turf (normal battles online), it's harder to do this. The most common mistakes I see in normal battles are uncoordinated pushes onto the opponent's side, and playing for splats instead of map control.
Above all, I'd recommend playing a weapon you're familiar with. There's no sense putting yourself at a disadvantage with something you haven't played much. Most shooters, dualies, and splatlings play well in Turf because of their painting ability, but some sloshers (e.g. Bloblobber) and even chargers (E-liter!) can paint well also. Specials like rain, Bubble Blower, and Bomb Rush work well in Turf. I agree with others that Last-Ditch Effort is good, but play any abilities you like.
~A.