The trick to not getting plundered... is to simply have nothing to plunder in the first place!
When I research the final tech in an age, I usually 'camp' in that age (by not researching techs of the next age) to level up my GBs and collect new ones. Why? Because some GBs offer military boosts which increase the attack and defence of an army. Whether the boost goes to the attack or defence army is determined by the GB itself eg. Cathedral of Aachen dedicates its bonus to attack armies.
Since you can't beat him, I'm assuming he has a pretty decent battle bonus from his GBs (or special buildings). I would advise you to try and collect a battle bonus.
But remember that collecting on time is still the best and most versatile decision to make.