Just to throw my opinions out there:
Go for a Hidden Power.
I think HP Rock/Ice should be considered as this thing would have nothing to hit Flying Types. Rock probably edges out a bit more to be able to Hit Fire types as well. Ground and Fire are viable as well.
Timid vs Modest is debateable. Not too sure about Unburden though as it requires some rather specific items that Accelgor will likely not want to run. Only exception being the Focus Sash.