Best way to destroy starbases

Sounds intriguing. I have not played Entrenchment so I have no idea. But are you sure you didn't hit mines? And what level were your cap ships?

Well the Advent mines come from Soviet Russia, they hit you, but that isn't necessarily the reason for a failed attack.

Fully upgraded starbases are monsters.  Your best defense is to stay away, try using bombers and long range frigates, and stay out of range the best you can.

On single player I like to use a big group of teched up LRMs

As Advent, the best strategy is to simply stay out of range while your strikecraft reduce the base to slag.  Well, that works for every race, really, but the Advent excel at it.  If it's a Vasari starbase and it comes after your carriers, I try to stay out in front until its about 1/2 health, then just let my fleet finish it.

As TEC and Vasari, I can usually just fight it out with my fleet, being careful to pull back any severely damaged ships, while my strike craft hit the repair bays.  I don't think I've ever lost more than 3-4 frigates, if that.

I don't really use siege frigates.  Maybe a few, but with SBs being so weak, you don't need them.

I built 4 marzas and sent them to destroy a fully upgraded AI starbase (vasari) when they near ran out of health I sent them away by the time all were gone the starbase was so weak my normal fleet could kill it plus capital ships gained exp.

I built 4 marzas and sent them to destroy a fully upgraded AI starbase (vasari) when they near ran out of health I sent them away by the time all were gone the starbase was so weak my normal fleet could kill it plus capital ships gained exp.

I don't know if I hit mines but after I was in firing range and was no longer moving I was still taking heavy damage and probably using more strike craft and LRM's would help. I will try it out  and reply again