You can save and then load to confirm their difficulty. It will show you the difficulty of all AI's (and, in fact, allow you to change them) on the game load confirmation screen. I do not know how changing an AI's difficulty when you load will affect achievements.
I will say that some maps can confuse the AI somewhat, so that may be what's causing the issue. As well, the AI tends to be a strong rusher (due to its resource cheating) but sucks mid-game because it doesn't research technologies very fast, so you can easily outpace it. If this is a multi-star scenario where it can't rush you, it may have built a huge army and entered upkeep even though it's too early to start fighting. They also tend to get their capital ships killed off, so often times the player will have a fleet of 16 high level caps and the AI has a handful of level 2's and 3's.