It depends on what race you were
If you were TEC, the answer is Flak, armor upgrades, health upgrades, combined with hoshikos if possible. In general, TEC has to outnumber the Advent to stand a chance.
Vasari can get enough Assailants early that they can effectively counter Advent. The nanite attack the evacuator has is a great counter to advent abusing shields. You can micro target the enemy cap and take it out very effectively.
Advent should be doing illums vs. illums. I know that's just rock vs. rock, but that's all I've seen. What's key is pickin the right moment to get guardians, keeping ur shiels higher than your opponents, and if possible, getting a halcyon to accompany ur mothership
These are my early game answers. You said 20 illums so I'm figuring you were rushed and this was early game.