I'll kill maybe 9 or 10 of em before I run into drones or vessels and then I start to lose drastically.
Run away and get a bigger force. Make him attack you so that you will have the advantage of turrets, hangars etc. Sentinals make a great meatshield against illums.
Scout him so that you can find out if he is rushing to get illums. If he has 3 mil labs early, then quickly attack him with scouts before he gets too many illums. Scouts kill LRMs, but getting enough to counter 30 illums is a waste when he starts using something else. That why you should attack with scouts before he gets so many.
I played last night as TEC, ran into an illuminator fleet of about 30, with my 29 LRMS and 12 or 13 flak and 5 hoshis I lost. I played another game where one of my allies fed the other up to around 50 illums and he engaged a fleet of assailants of about the same size, and came out victorious only losing around 15 ships
Well, the other guy did lose half his entire force. I would not have brought LRMs to that battle. I would have got more flak, more hoshikoes and Kodiaks if possible. A Dunov with EMP charge is a good investment against shield-reliant Advent. Phase missiles are even better.
Use the Egg or Marza to kill the Mothership. If you take it out, they will suffer a big loss. If they have a Halcyon kill it straigh away. Its fragile so that should not be too hard.
Do you really need a fleet of Lrms 3 times the size of the illuminator fleet to beat it?
You do need a bigger fleet of LRMs to kill illums, because LRMs are not designed not counter LRMs. The stronger LRMs (illums) will always beat weaker LRMs in equal numbers. Use scouts, flak, carriers, HCs etc. to kill Illums. A skirantra carrier with repair cloud helps alot too.