Ok first unlearn the lessons u think hold true.
Don't FF (focus fire) against a late game advent fleet especially if they have more than a couple guardians. The repulse will blow the ships in an arc the ships will then go for the FF'd ship and the transversal angle will get you RAKED across the illum field of fire, meanwhile theres a chance your not even going to get a shot off at the ff'd target if repulse pushed you out of range. This holds doubly true for LRMS/Assailants, they are your main defense vs repulse cause they can outrange the effect but not if you ff a ship and the guardians are in between the lrms and the ff'd target (like a cap). Know where your stop button is a be ready to use keep your lrms at the max range possible and if he splits out the guardians then take them out before moving your ships any closer to the clump.
This goes for all games of any type vs any oppoenent but specifically useful vs a late game advent clump fleet. Don't just jump into the gravwell and run straight for the fleet, plan your jump, jump your ships together, start them out in hold position so they dont start running off individually and get picked off. Where u jump from in one gravwell determine where you come in in the target gravwell use this and get as much space as possible. If at all possible split your fleet in two and jump in at different parts of the gravwell.
Back again to point 1, guardians. This should be your main priority target and you should have enough space when you are engaging an advent fleet to allow your bombers to engage and destroy as many guardians as possible before you engage. Oh yeah have plenty of bombers to take out guardians
Also you can use the intercept ability of kodiaks to run through repulse, a small group of 10 or so heavy cruisers on a seperate group key can be very nice to rush in and take out guardians if your micro is good enough and you can use the bombers and the cruisers guardians should be to much of a problem.
As much as I am going to hate posting this on the forum cause im going to kill myself as an advent player heres tip number 2. Kill the mothership first after the guardians are down. Micro off some units to kill any domina's. This is really really simple the mothership is the primary healer and the domina's are the secondary healers these two units are the reason advent take such few losses in engagements. Even if you lose the advent player is going to be hard pressed to hold or push any advantage he has gained with the main heal support for his fleet gone. Even if you just chase the mothership away that has now given you a much better opening to start putting a dent in his fleet.
If the advent player has a level 6 radiance and a mother ship your going to have to micro your butt off as well, you shouldnt have anything that even resembles a clump your going to have to spread out as much as possible, set a fleet to all loose when you jump in then band box and start moving ships all over the place cause cleansing brill/malice will wipe out 10-20 ships at a time in a clump (it'll take 10-20 ships down to almost nothing then the advent player will issue a move order and stop FF and free fire them down in appx 5 secs).
Now on to the hardest part about advent killing the illuminators, FF again is not the answer here cause the might be illusions, you order your whole fleet to FF an advent illum illsuion and youve just wasted prob a few of your ships for absolutly no gain. Unfortunatly this is where advent is going to win and you kinda have to eat a larger fleet loss (which your econ or RA should handle), micro off units and FF down destra's, disciples, aeria's, caps or whatever else you possibly can and hurt him as badly as possible again your econ can sustain the losses the advent player might be able to but shouldn't be in the same position you are economically.
Strategically you should be creating smaller fleets, a couple hvy cruisers, a hoshiko, a cielo, maybe a few lrms, and attacking the advent player all over the place. Advents strength is in one huge massed armada if you force him to break off ships from his main armada you can significantly increase your chances at beating him in a primary fleet engagement.
Basically advent does has the toughest end game fleet but RA and TEC's econ can beat them but you have to micro more and focus on engaging an advent fleet on your terms even if you have to lose a planet to wait for an engagement you know you can at the very least hurt him in.