As we level up, it takes more and more exp to gain another level. However, so far (up to enemy level 7), each enemy gives the same amount of exp (1), even though higher level enemies take longer to defeat.
Suggestion: Scale exp gained from defeating an enemy in the campaign with the enemy level (example: lvl 1/2: 1 exp, 3/4: 2 exp, …). Currently, it's “better” to keep farming lower levels instead of fighting enemies of appropriate strength.