Starbases.

AI will run right into your defended planets. Eliminate their fleet then mop up.

Also, I would recommend going to equal number of unfairs as a stepping stone if you're having problems with more AI opponents. You'll get an idea of how to handle more ships than the hard version, but not as many as having an extra opponent.

For general strategies/tips, there is an excellent (pre-entrenchment) guide on this forum which I am too lazy to go find and link at the moment.

Do you know what this pre-entrenchment guide is called so I can track it down?

I wouldn't know what to look for otherwise!

Thanks for the tips so far

A good solution is to quickly tech up to long range frigates and then rush.  The AI will be producing light frigates, and it will respond to a rush by pumping even more light frigates, which your long range frigates counter.  Whether you defeat them or not, your capital ships will gain a lot of experience and you may quickly get high level abilities.  If you have a Marza that reaches level 6, the AI is pretty much dead meat.  However, a high level Progenitor is also a great asset if it's backing up lots of illuminators.

The AI is pretty decent if you leave it to its own devices, but if you start attacking it, it just doesn't know how to balance its expenditures between military, economy, and tech, and usually just dumps everything into low-level military units and gets killed.