Guild Wars 1 had this figured out.
Everyone could Zerg a target and it would die super fast.
You could counter by applying Aegis or Guardian
Aegis & Guardian (GW1 Enchantments similar to GW2 Boons) blocked 50% of Attacks for the entire duration it was active, not just 1.
This countered Zerging.
Then to prevent Aegis/Guardian from being overpowered, there were other abilities to prematurely remove Enchantments.
There were also abilities that were unblockable to “circumvent” blocking mechanisms.
I suggest GW2 Aegis be changed to block either 50%, 75% or 100% of attacks for its entire duration instead of blocking only 1 attack.
In addition to make this fair there should be more abilities (which classes would need to be a design choice)
-Abilities that Remove all Boons on a Target
-Abilities that Remove Aegis from Enemies in a Radius
-Abilities that make your next X attacks Unblockable
-Abilities that make Allies in a Radius their next X attacks Unblockable
-Abilities that are Unblockable
This would increase skill required because simply calling a “T” target wouldn’t be good enough anymore
Defensively your team would need to manage abilities that counter Zerging, Targeted Aggression, and Attacks in General.
Offensively your team would need to manage abilities to Maximize Zerging, Targeted, Aggression, and Attacks in General.
Lastly yes there are other other non-Aegis methods of blocking but they simply are not enough.
When an allies block ability ends and they go from full HP to downed in 1-2 seconds in a group engagement I would say that is a little excessive.
The Key to being balanced is that these changes need to be properly implented on both Defense and Offense.