Buzzkill DM: Locusts, even under the loose control of a druid, are not intelligent enough to be "willing" to be transformed, so they can't be targeted by animal shapes, rules as written.
Better DM: That's cool as hell. Do it.
Best DM: I'll allow it, but you better start watching the skies from here on out. If you can do it, then the enemy can too.
Thinking quickly, the BBEG casts Berserker and Shield Bubble as AoE spells in the sky above him.
While he does suffer considerable damage once they all hit the ground, the enraged elephants do not. Now the battlefield has hundreds of larger-than-life bucky balls rolling in every direction.
Each player must flip a coin before every turn to determine if they successfully dodged the latest pachydermian steamroller barreling towards them with reckless abandon.