So, if you are set on giving another level to a character, why not use the formula for the level via experience and grant the appropriate experience given the level using a calculation environment to change the value dependent on the character level?
<DisplayName>Experience to Next Level</DisplayName>
<Description>Experience needed for this unit to reach it's next level.</Description>
Not sure if this unit stat is functional, but may be helpful in your calculation as the value would be
to give the unit the amount [UnitStat_ExpToNextLevel]
Then you should gain a level. But not sure if this is functional at the present state of the game, but could be.
Edit: Poor mans way is to just grant the experience necessary to reach the next level given start of the current level.