Hard Too easy?

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.

Also,  When playing with FFA, were the seven AI all on seperate locked teams?  If so, I find those games to be much easier than say FFA 1 V 7 AI with unlocked teams. 

With the possible exception of a 3-way game, locked teams is always much easier than unlocked teams games played with the AI. 

With locked teams, the AI will prioritize its attacks against its enemies without prejudice as to whether it's a human player or another AI.  In other words, a level playing field.  With unlocked teams, the AI will almost always prioritize its attacks against the human player, and you need to complete missions to convince them to do otherwise.  This adds a layer of difficulty just to get the level playing field that you get by default with locked teams.