You missed the part where the Lucifer was said to be immune and not merely resistant to GTA weapons.
I believe the exact working of the briefing was "
appears to be immune, not merely resistant, to all of our weapons"
Yeah I'm pretty sure that getting every able spearman in Neolithic France isn't going to stop an MBT
Not really comparable, given that shield is not really armor and plasma guns aren't spears. But if you want to get technical, bash a armored vehicle with a hammer long enough and hard enough...and it WILL get destroyed. Sure, it might take 10000000000000000000000000000000000 hits, but still. Even stone gets eroded by wind and rain with time.
Or if you want another example, how about rolling a massive boulder on the vehicle? There's a limit to any armors endurance.
NOTHING in the universe is truly invulnerable and indestructible. Such things simply don't exist.
Again, action and reaction. Conservation of energy. Thermodynamics. Pick your poison.
It's simple logic at work. Stopping destructive energy requires effort (counterforce). Effort = power. Power is not infinite and has to come from somewhere.