Then use wallwalk…
Seriously, the basi is quite difficult to master, but if you do, it is an extraordinary weapon. Personally, when playing human, and I go out, I don't fear dragoons or marauders, for I can outmaneauver them, but a basilisk – forget it! One touch, one kill, if you are not lucky enough to be able to free yourself.
All in all, I think the problem is that certain people still mistake Tremulous with a classical brainless shoot-em-all–game, in Tremulous, you are required to THINK out your opponent as well. A hint for basi: Use wallwalk.
As a rule of thumb, everything that can walk on walls, should walk on walls, and if you have trouble with movement, just /devmap ATCS or something, put some unpowered turrets around in the map, and then try a basi circling around them. You'll figure it wont be too hard. Later, put a repeater near the turrets and learn to kill an actually powered turret, if you master this, join a game and try it on the real thing.
Get skills, man!