This soon leads to the old Fire Emblem trick of parking all your characters in the enemy's personal space to keep them trapped until the unit you want to get the kill comes along.

Having the lion's share of XP awarded for the killing blow leads to one of two outcomes. People play gamey to equalise it or they start to rely more and more on whoever runs away with the XP lead the first few levels.

It may be better to have the XP all go into a shared pool and let the player spend it between battles (or have a catchup mechanism using another resource the player can reasonably obtain like Warriors games have the training ground).
Oh yeah.. while i love the creativity found in for example the disgaia games, then i also still hate the killing-blow bit of xp management with the fury of a thousand suns.

As this gentleman mention it easily leads to a downward spiral of one character hugging all the xp constantly, and makes it a bitch if you want a even partly equal team.