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Anti-Armor seems the closest to what I'm describing, but deals with things which are specifically good against armor, not just bulky targets in general.
You could also approach this from the method they use to deal with the bulky target. Since you're talking health pools rather than armor, anti-large-health-pool units often use a Percent Damage Attack (see paragraph 5 for the variant on percent of max hp), or a Gathering Steam effect where the longer they stay on one target, the more DPS they deal (whereas they can't gather steam against a swarm).
There is also Mighty Glacier for, in general, slow attackers with an extremely hard punch.
Edited by KD

A character or item which is most effective at taking out highly-durable targets, but worse against huge hordes or frailer opponents. I've been working on a draft for a trope like this, but I'd like to make sure this doesn't exist first.
The Battle Cats: There are some units, such as Balrog Cat and Lufalan Pasalan, which have immense attack power balanced out by a single-target attack. These cats can easily destroy bosses with huge health pools, but can be overwhelmed and defeated by a Zerg Rush of weaker enemies.