In Easy Mode, all attacks do 1.4x times the "normal" damage to Magnus Barriers, so that might be part of why it seems so fast. You can kill anything in Easy Mode REALLY quickly if you know how to do a lot of damage.
Also, I'm not sure if this is what you meant, but your comment lead me find a bug (or more like unintentional behavior) with the Royal Arcanist's defensive spell, so thanks! :)
The "intended" way to use the spell against Magnus Barriers is for you to cast 3 debuffs, then reapply them right when they wear off. The "Death Toll" debuff is the most powerful one, and doesn't do any damage until it wears off, which means it's negated if it's overwritten. But, it looks like the "Pain" and "Toxic" debuffs are actually "ticking" too soon after they're reapplied, dealing extra damage... which makes spamming the move just as effective as using it carefully, which isn't what I wanted. So, I'll fix that too.