1. PvE viable
2. Reliable DoT effects
3. 70-100% DoT uptime
4. Any element
5. Melee or range is fine
The game technically don't have damage over time but rather damage over turns (Burning). Damage per AP and/or MP is another to consider if it is effective in a PvE setting.
Sram being the highest in it's damage proficiency, pandas coming in at a close second, and Sadi falling behind them.
The difference of course between the 3 is role and team cohesion. Sram has the most uptime though.