Best to ask this in the strategy section. Otherwise they may not see it here. Not to mention that 'here' (Off-Topic) is spread across 12 different sites, some unrelated to gaming at all.

First off, I suggest locking teams. AI will always, ALWAYS get friendly with one another, and then begin sending huge, joint fleet attacks against you. In one game, (albiet many patches ago) I had a game with unlock teams, all AI set to unfair difficulty. Within a couple hours, I had about 6 of them sending huge waves of ships against my planets; I only survived by fleeing to a neighboring system, barely managing to colonize a new planet before they completely owned the 6 or so I had. So make sure to lock teams or you will get ganged up on like crazy.

I personally find an all or mostly capital ship approach (for non-entrenchment, which I am presuming you are talking about since you make no mention of version) works well against all but the biggest combined AI attacks. IMO most of the strategies are more or less whatever suits how you like to play. I usually build up all 16 capital ships before building more than a handful of frigates. There are a few good reasons for this. First, they are strong enough to beat off most attacks if used properly (micromanaged). Second, building them early gives you a big advantage when it comes to their levels. Third, one of the most important reasons, is that unlike frigates, they do not go down easily and so do not need to be replaced nearly as often, giving you more funds to spend upgrading planets or researching. From there, I mostly turtle my way through the game, only taking one planet at a time, keeping chokepoints to a minimum, and then building up the outer planets' defenses. During this time, most of my funds go to teching up, which is important for keeping the capital ships alive. make sure to make teching up antimatter generation a priority with this strategy, since that is what keeps your ships healed (via whatever ship has healing abilities for the faction you are playing as). The downsides to this strategy is that if the AI decides to send multiple capital ships in with their normal frigate spam early on, you may be forced to retreat back a planet or two. The other downside is with only capital ships with no supporting frigates, there is very low dps output. Battles will be very long and drawn out (IMO thats the best kind) and you will be stuck turtling through, since you just won't have the firepower to destroy their defenses until about the time you have the entire tech tree researched and the capital ships are higher levels. Usually by the time I have all 16 capital ships, I am about ready to go on the offensive; and by that point the cost of a new capital ship is minimal, since you have a good deal of the civilian tech tree researched.

Edit: to adapt this approach for Entrenchment, use fully upgraded starbases to defend your chokepoint worlds, with the worls bording them having partially (25-50%) upgraded starbases in case a fallback position is needed. This leaves you able to use your entire capital ship fleet to attack, and absolutely nothing the AI does is able to stand against a moderately leveled 16 capship armada.

Edit 2: Just so you know, I Vasari. Jarrasul Evacuators are my favorite ship in the game! When full upgraded they can colonize, drain planet, use nanites which do 1200 damage over a few seconds (one shot kill for most frigates), and when 2 are used together with a phase inhibitor can stop enemies from escaping altogether with their gravity warhead!

 A few tips for outsmarting the rather dumb AI:

1) AI will always attack defenses. Be sure and build-em around all your border worlds. AI is particularly fond of sending a bunch of siege frigates to take poorly defended planets on your borders. Be sure and upgrade emergency facilities and put up turrets all around the planet to kill em. Hangars with fighters are particularly effective at killing these guys. Hangars with fighters are also great at taking down enemy assault cruisers(entrenchment only)

2) AI will ALWAYS run when outnumbered, follow him and make sure you turn that fleet into debris. PJIs will slow his escape.

3) AI will almost always retreat to defend their HW.

4) Planetary militia are dumb as rocks. There are 3 things they are guaranteed to do. 1) they will chase scouts fruitlessly, sending one in beofre your fleet means they wont attack your fleet until the scout is dead. 2) Siege frigates will break off their attacks if the planet theyre protecting is colonized, kill them first, colonize and slap up a couple of turrets to handle the rest and youre golden. 3) All the militia ships will go after your colonizer frigate as soon as it enters the well, be sure it has plenty of AM to spare when jumping it in with militia still around so it can colonize and jump out without delay.

Other tips:

Dont stop colonizing as long as their are uncolonized planets to grab. If your fleet is needed elsewhere to defend, build a small secondary fleet to continue grabbing planets. LRFs are great at subduing planetary militia.

Grab Asteroids! Ignore dead ones. Asteroids are easy to grab cuz they'll always only have 2 ships defending them. Dead asteroids should only be colonized if you need them for tactical purposes, otherwise they are a waste of your precious resources.

Dont forget about your economy! AI always gets an economic advantage to counteract its lack of smarts. Build up your econ to keep up and you'll always clobber him with your tactics.

Good luck!

Build Hoshikos.

Also, don't get more cap ships until you've at least upgraded your fleet twice (or are about to do the 2nd). Ships are way more effective than caps in the early stage.