I'd be worried about the low hp combined with fire vulnerability. 70 damage from fire and the everflowerer is dead in one hit. Thankfully this is higher than even a maximized fireball, but with 7th level spells and the prevalence of Orb of Fire (dealing 13d6 at this level or on average 45.5 damage; or 67.75 damage to the everflowerer) it'll be lucky to last 2 rounds; he might last 2 rounds if they go after the barksinger first and his rage activates (also mistake in the text has it say he gets an enhancement bonus to Rage). His attack bonus is dreadfully low without Rage + Barksinger (+17 when norm for CR 13 according to the CR assessment text in MM is +21). His AC is also pretty pitiful (and has another mistake where his flat-footed AC is 3 higher than his normal AC). Finally he dies if they deal 34 damage to the Barksinger, no difficult task for Lv 11+ characters that should be facing a CR 13.

tl;dr: everflowerer is probably over CR'd.