Level 20 Lancer can raise their damage 15%. Can not do any aoe damage at all.
Level 20 (70) Dragoon can raise their damage 15%, can increase the target's piercing damage taken 5%, can periodically raise their damage a further 15%, has a dot, can raise the party's crit rate, can raise their damage yet another 10% while raising another player's damage up 5%, has multiple aoe attacks.
There is no reasonable way to make these players even remotely similar in dungeons.
Here's a simple solution:

Raise everything's HP relative to the levels of the players in the party. Also, lower or raise the tank's net enmity gain by a percentage based on the level of the players in the party. Finally, raise the incoming damage done by enemy units by a percentage based on level of the players in the party. The exact percentage would have to be a little different for each stat change, and also depend on a slightly different equation. But it wouldn't be that difficult.