I did, finally, get to annihilate a planet. It was literally the last thing I did before I quit the game to write this post. It was something like my third or fourth attempt. On previous occasions, I'd gotten pretty far along in the process of building a superweapon when the enemy commander just happened to wander into my line of fire and get destroyed.
Of course, that's probably my just punishment for attempting to delay the end of a math purely to deliver a superfluous, planet-destroying coup de grace. Luckily, for my last match, building the giant laser really was the most efficient way to win. I was on a system with multiple planets, facing multiple commanders, and it was either blow up an enemy planet all in one go, or launch a complex interplanetary invasion. The laser saved me the trouble of having to scout out the enemy's location.
However, I think my victory was only possible because the AI on normal mode was really weak. Not once did I have to repel an attack on my base, and for almost the whole of the galactic war, I was able to seize mineral deposits virtually uncontested. Only when I accidentally strayed near the AI base was I in any danger of having my units destroyed. It was useful for practice, and I think I've got a pretty good handle on the tech tree, but it doesn't make me feel especially proud of my skills.
I've been told that the AI in skirmish mode is stronger, so I think I'll give it a try next. I'm not sure how hard I want the game to be, exactly. One of the toughest parts of the genre for me, after the annoyance of having to divide my attention, is that I don't like losing units and structures. It's a weird reaction, I know, but having built those cute little guys, I hate to see them wrecked by my brute of an enemy. I wish I could play the RTS without having to labor under the threat of war, but then, what would be the point?
Since Planetary Annihilation is not a city-builder, but rather a war game, I think I'm going to have to accept some level of destruction if I want a decent challenge. I guess I'd want to find the hardest difficulty where it's possible for me to expand slightly faster than my stuff is destroyed. That will likely take some fine tuning.
Still, I'm halfway through the game, and it's been relatively painless thus far, so I'm optimistic about the back half. Even if harder difficulties prove too frustrating, I'll still be able to kick it back down to normal and coast my way through my remaining time. It may not be dignified, but it's an approach that works for me.