Well, that's the problem of giving the planet killer only firing point. In order for it to be effective, the target would always have to be in front somewhere. In the original episode, the Enterprise was firing over it and heading down toward the back. Obviously the planet killer wasn't shooting at it then, so it's just as plausible for them Enterprise to be strafing it from the other direction. Probably more plausible since they are now heading back into the planet killer's field of fire. Nah, they were just below par effects. The Enterprise wasn't firing "weak phasers," the PK was just too strong to be penetrated by them. That distinction makes the machine that much more formidable. There was no power in the beams, no sense of reality. Even earlier episodes had a glow around the beams at times, giving them a sense of energy. Plus, orange? Never before or since. They weren't the worst phaser effects in the series (Arena has those), but they were really cartoony.