Nefashu

Declaring this as an attacker triggers the shrink, and that single design choice (an attack trigger rather than a tap ability or an enters-the-battlefield burst) is what gives it teeth, because it recurs and costs nothing beyond the willingness to send it into combat. Where most repeatable -1/-1 effects on a body ask you to pay or sacrifice each turn, here the price is simply committing to the attack step, and the spread of targets clears a wide swarm of small creatures the way a one-shot edict never could. The math against go-wide boards is unusually lopsided for a creature of this era: -1/-1 to five separate bodies wipes five one-toughness tokens in a single declaration of attackers and can reset a tribal swarm before it ever swings back. The fragility is the counterweight. A 5/3 dies to nearly anything pointed at it, and because the trigger fires on attack rather than on damage, the shrink resolves whether or not the opponent blocks: send Nefashu in, shrink the defending creatures, and a small blocker may now die or fail to kill it, but a single large blocker simply eats the trigger and trades during the combat damage step, killing the 5/3 outright. The shape of the play is asymmetric in both directions: against tokens it is a recurring sweeper stapled to a clock, but against a single fat blocker the trigger does almost nothing while the body walks into a fight it loses. The reward for attacking is enormous; the body that has to attack to earn it is the thing keeping it in check.