Yeah that's fair, it would be useful against buffed up first attacks but overall, if they are going swarm it's quite likely they will have more than one and unless you have an infinite defense, it will still only reduce a small amount of incoming damage.
I would argue that killing the things that make them strong (Straywood scout / marked for death / etherian lifetree) would be more beneficial than trying to get defenses overall.
In that regard I think your best bet is to get into the zone where these things are congregated and prepare a zone attack like you suggested because those seem to be the most effective way to deal with swarms as you said. But of course, the enemy mage using swarms will be aware of that too and try to thwart you.
Also, congrats on the tournament, looking forward to the update!