What is a Sentinel (Buffer) dragon good for in Dragonfire?

The Sentinel is the Buffer: key stat Instinct, tactical damage. Its job is to inflate your allies and to heal.

Its auras reinforce your line — Wise Vigor (+20% Instinct, plus Recovery) — and many Sentinels carry a Recovery that restores troops (Malachite +70% to 3 allies, Dawnseeker +30%). On offence they hit in tactical damage, which rises with the attacker's Instinct and falls with the defender's Intelligence.

Syrax, the Legendary Sentinel, is a case apart: she switches on the fire of the whole line and hands out First-Strike. Put a Sentinel in any army that has to last (multi-wave attrition).