So far it seems like 10exp is the base for engagement, and you gain an additional +5 for not getting hurt yourself, or the base is 15 and you lose 5 for getting hit. And the whole net gain is diminished by internal level. I would need much much more testing to determine exactly what rate ILv lowers this, so instead I'll come up with a slope of my own.

[quote=PlatinumSkink]
I'm not sure how you're usually playing, but I am pretty used to getting exp from hitting enemies, so... yeah... haha... Maybe it is just because I play on Normal all the time... ... ... I just felt like mentioning.
[/quote]
On Normal, the ILv cap is 20. This makes the minimum Exp income possible higher than that of Hard mode. On Lunatic the ILv cap is 50, making the lowest possible Exp gain smaller than Hard mode.

On Hard mode, the lowest possible Exp gain from an engagement where the unit kills the enemy is a net earn of 8exp.