Same here, they become fast as hell and one hit the door, if they enter the coffer room they can demolish the coffer e ruin the purge. I there is only the coffer in the room they still can demolish it.
Plus if you do a narrow entrance to the base and put a siege galdron, when the galdron drops the oil in fire they will consider that they cant reach the coffer and will retreat.
If you block the corridor with your body or being on a mount, they will give up and retreat. They really want us to provide a free way to attack the coffer and wait they gather inside to figth the entire mob and get overcrowded and die? lol the purge want it easy.
To finalize they can simple pass through you and your mount sometimes. And a single foot soldier can simple quiclk you in your mount and make you fly far away.
The new purge is very fun, but those issues killing it. The most frustating to me is the purge giving up because they want to face punny defences.