In my experience on the old Teen Grid, the most effective and most widely used tactic is the ban wall / no access list. If someone forgot to put up the ban wall, then usually somebody pulls out a railgun and spams the sim with megaprims.
On a slightly more serious note, although I was never involved with combat myself, I picked up what I call "Prim Jumping". Essentially if you run towards a prim that is about .5 m tall and jump just as you hit, it can launch you much higher into the air than a normal jump. Just plop down a plywood cube in a sandbox and practice!