I have been on more than a few panels on how to create good military weapons in SF. The key word for me in these cases is credibility. The reader has to believe in it. This filters out the death rays and other mega-weapons favored by SF melodrama. Sorry, no galaxy-cleaning device. No one-click world annihilator. Gone are the God-killing devices that can mow down entire armies, and even the battle cruisers large enough to engulf cities. Yeah, you can find all of these, but not in anything I write.
True, you can probably create something that washes cities aside in one blow - we have those today. What you miss out on is the human element. The man-and-machine staple for science fiction. In truth, you really don't want these kind of weapons. They tend to be wielded by props instead of characters which only extends the lack of belief from the weapon to the person using it.
So, there is my case for not having the ultimate doomsday device in your hands. So what should you be considering? I have a few loose rules I try to adhere to.
1. The weapon must have limitations. Be it fuel, ammunition, cost, or environment, the weapon should be constrained. Often this is expressed in range, rate of fire, or effectiveness. It shouldn't be able to kill everything, and should be subject to countermeasures that render it either ineffective or useless.
2. The weapon should be base lined on something that exists. Plasma cannon derived from today's heavy machine guns. Probes derived from the abilities of today's unmanned aircraft. The idea here is to anchor your weapon with something your reader can already relate to. A self-aware ball of gas floating from target to target is not going to be as easy to visualize with as a robotic flyer doing the same thing.
3. The weapon must cost the user. Be it weight, painful reaction to using, or even psychological distress, your device should be something of consequence - the "shadow" grounding the weapon and user with reality.
4. The weapon should have a weakness. It can be jammed, broken, or simply fail. Not saying that it would do this all of the time, but there is something to be said for a plasma cannon whose barrel falls off when over-heated. You can use your environment to induce failure - or at least force the unlucky soldier to clean or otherwise maintain the thing to keep it from killing him.
The tough part of creating any weapon is in keeping up with our own advancing technology. One-shot, one-kill weapons exist today. Others can get you even from behind a wall. We're rapidly coming up to energy weapons that you won't even see when they take you down. What does one do when a realistic terminator can identify, release weapons, and kill Sara Conner in the blink of an eye? I don't care how fast Reese is with that shotgun, she's gonna die. Your defense in these cases will have to be a form of countermeasure. Something to thwart targeting. Something to deflect the projectile or beam. A way to slow down or incapacitate the shooter before it even arrives at a firing solution. Guess what. Any device you create for this purpose must itself be put through the "rules". Counter measures must themselves be locked down by limitations.
An armored suit might cost more than the soldier, for instance. It may have an unacceptable fail rate or be easily tracked. How long can it run on power? If you blind its electronics, then what? Make armor effective against one type of weapon, and the enemy simply changes weapons. Make it effective against all weapons, and now your soldier can't move. In the end, the practical and economical solution will be to outfit the soldier with something less than this suit but still capable of meeting the widest range of threats. In this scenario, which I subscribe to in my novels, the idea is to avoid what you can and get hurt the least by what attacks you before you can kill it. This puts the element of risk, and drama, back into the scene.
I put this theory to the test in Rogue Dancer where Mikial thinks the armor and cloth she's wearing will keep her from being seen. She has on camouflage that would put the Predator to shame. Unfortunately, she has no clue about being silhouetted against the background radiation of an old crater. Oops.
Realism. For every "Ha!" there's an "Aw shit!"