Flamestrike
Legend
It has wings, so I guess dropping it prone would make it fall into the river. Good opportunity to figure out how bad falling in is.
Shooting the east one with the hand crossbow:
Attack 1
To Hit: 1d20 (2) + 10 = 12
Damage: 1d6 (5) + 18= 23
Attack 2
To Hit: 1d20 (10) + 10 = 20
Damage: 1d6 (3) + 18 + Trip Attack d10 (10) = 31, STR save 18 or fall prone
Attack 3
To Hit: 1d20 (18) + 10 = 28
Damage: 1d6 (5) + 18 = 23
Attack 4
To Hit: 1d20 (6) + 10 = 16
Damage: 1d6 (6) + 18 = 24
If it does fall in the river, just use the last two attacks on the one that got hit by the Firebolt, and regardless move to next to the one attacking the Rogue.
It passes its Strength save, and is still up (and flying).