Ok well Advent gets 6% mitigation with culture from the civic tree, 2 techs at tier 2, 2 at tier 4 2 at tier 6. Thats 6 techs plus temple of communion research, 7 techs! Then you need to get tier 6 in military by getting 2 research at tier 1, 2 at tier 2, 2 at tier 4 and finally 2 at tier 6. The last 2 tech will give you 2% each, for 4% total. So you end up with +10% mitigation after you research 7 civil and 6 military techs. Then to shield your buildings you need to research hanger in defense tree, and another tech that gives near by buildings 750 hp shield. We are up to 15 techs needed so far, now you mayw ant to get one more tech which gives additional 500 hp to turrets and stacking buff, so after 16 techs you have very tough turrets.
Most of that is irrelevant and icing on the cake. You only need one tech: Synergy. The others make it a bit more of a pain, but Synergy is the main culprit and has no pre-reqs.
Now if you are TEC you can research your Cielo crusier prototype, and the designate target ability, 2 techs, and you give target -25% mitigation.
If you are Vasari, you can get simply Subverter prototype, which comes with it's ability Shield Disruption that gives you -10% shield mitigation and -25% Phase missile Block.
Cielos do damate taken, not mitigation. It's also 40%. And the problem with both of those "counters" is that they aren't really good at it. Cielos are way more effective on un-mitigated turrets and even not needed, and Subverters don't do anything. On mitigated turrets they make killing them faster, but it's still a lot slower than un-mitigated ones. Not to mention the Subverter also requires the full phase missile research line to be any sort of effective. And what do the Advent get?