Laser

Laser Weaponry is extremely damaging to unprotected ships, but is quite easy to protect against due to the fact that it's literally weaponized light.

Effectiveness
The effectiveness of laser weaponry depends on the intensity which has a couple of factors that affect it (the power of the light and the area it hits). With just the right balance of each of these, the laser can do serious damage, cutting though almost anything. If a minor alteration happens to any of these factors the intensity can drastically increase or decrease. One example is here: a laser that hits an area of 30m2 with a power of 5W will have an intensity of 0.2W/m2. Whereas a laser that hits an area of 3m2 with the same power has an intensity of 2W/m2, an entire order of magnitude more. Take this and keep the area the same, but increase the power to megawatt levels and you've got an extremely powerful laser on your hands. One example of a powerful laser is the HELL on the Power Of Arskodia II.

Countermeasures
For such a powerful form of weaponry, you may think it must be very difficult to defend against and you would be half right, and half not...

A laser can be defeated just by using a mirror, yes, but that mirror would have to be the size of an entire ship as lasers are very easy to redirect, only limited by the speed of the motors moving the emitter. A laser is also invisible until it reflects off something, like the armor of a starship and so a small laser that's fairly weak could be trained on a starship's hull completely unnoticed until they start losing internal pressure. The main countermeasure against a laser is Chaff Cannons, however, these can be bypassed by increasing the power to the laser and the area it will hit, keeping the intensity the same.