Gem

Proved to work in January 2022:
One trainer with this build can train two dragons at a time up with ease in Ghostlight Ruins.

Rally, then Scratch until you can afford Eliminate. All enemies other than bosses are oneshot kills with Eliminate as long as Rally is up. Prioritise melee enemies first (starting with fastest), then elemental weaknesses and typeless casters (for this one, Fire, then typeless, then Ice and Plague). Keep an eye on the turn order, and throw in "unnecessary" Scratches to build extra breath if you can. If you have excess breath, Sap can replace Scratch to help replenish lost health. (Do not Sap if your Breath is below 50 if you still want to be able to use Eliminate!)

Higher-level zones work otherwise similarly, but with the Rally buff up, each enemy requires one Scratch before they can be disposed of with Eliminate. Because of the slowdown, enemies get to attack you more.

Bosses will break up the system. Rally up and make sure to use Shred before it's the boss' turn. (Multiple uses don't stack, unfortunately). Keep building up breath with Scratch and replenish any damage with Sap as you can. The main goal is to outlive the boss. If you have excess breath, the cost of no-kill Eliminate may be worth it. Make sure to replenish bleeding on the boss and Rally on yourself as needed.

Oh, and play as if the dragon you're levelling isn't there. They're just along for the ride and to maybe tank a hit or two. Don't waste buffs on them. If they get a turn, just defend.