I basically resigned myself to not going first at the start with Nell, and just using Neris and Party Member 3 to lock down the adds and debuff the boss while Nell set up Earth Form buffs to tank a big hit. Neris's Lock and Load and Party Member 3's ultimate skill does great at that. (The adds are the biggest threat sometimes- the attacker's stun can really ruin your day. A Ring of Free Action is a good option on Nell for that reason alone.)
I think I did manage to beat that boss without the final detail once, but it wasn't easy- his second phase hits hard and hits fast. The Take Aim + Heavy Draw combo hasn't let me down yet, and both Fire Form and Air Form can really lay down the hurt. On that note, Tailwind Sweep in Air Form is very useful for getting the Agility advantage, just like the Pokemon move that directly inspired it; using it is one of the few ways to outpace the 250 AGI on the adds.