I lvl'd my bretto and knolan to lvl 22 cuz I had no idea how to exactly pull off the max lvl in one go but I learned from my mistake and dinged gully from lvl 2 to lvl 30 in one boss fight (gaining over 50k exp from the said fight)
What you do is simple, clear the last dungeon and before you engage the last boss, leave the dungeon via portal or walking, just make sure the path is clear to the boss so u don't engage loose mobs on the road cuz you want your boosted chars to be the lowest lvl they can be for the best exp gain.
Get out and switch ur grp "keep garrison as the main dps" and bring two chars to boost. If your garrison isn't that well geared it's fine, go around the world and pick up every single shrine buff u can find and drink from the well at town. Drink the best rum u can buy from the city (I think it's ambrosia) which grants 700+ ap, bring a Titan flask as well.
make sure u get all talents that buff damage and especially the talent that turns rage wave into piercing damage and all the overcharge talents + the 10 rage starter.
When the fight starts use enrage from garrison, use any ability from the other chars that buff garrison in any way "BUT DO NOT TOUCH" the bosses, and use the Titan flask on garrison from any char that doesn't have good buffs or any at all (make sure to keep 2 burst meters for garrison) when it's garrisons turn simply use relentless assault "lvl 2 burst, 60% crit" and cast rage wave. The other chars can defend or w.e but don't hurt the bosses so u can get the one hit wonder bonus exp from them. When it's garrison's turn he will one shot them all with rage wave and all ur low lvl chars will fully ding.