I'm not convinced that powerleveling will actually be a safer, more efficient method of gaining XP than partying. It all depends on these "other" party and combat features.
- XP cap for enemy level difference.
- Chain bonus for defeating enemies.
- Linked enemy reinforcements.
- Link bonus for defeating enemies.
From how it is described, linked enemy reinforcements are only called at the time the the character initiates the attack, and only once. Also ...
So if you're a solo level 1 character fighting a level 10 enemy, it's not likely to call for linked reinforcements to deal with you. So your link bonus for the encounter will be 0 XP.
A party of eight level 2 characters against that same level 10 enemy would likely cause it to call reinforcements ... that the party is then able to defeat for XP and then get bonus linked XP based on the number of links.
It depends on the numbers, I guess.