Board Thread:Battle Nations Discussion/@comment-173.78.222.200-20140907161906/@comment-11090687-20140907170840

They drop 100% (3 Skulls each) from Raider encounters with one of the Raider Luitenants (Sarin, Tronk, Crazy Blades). These encounters only spawn on outposts of players in the level range Lvl.30-Lvl.50, so check the outposts of in-game friends you have that are in this level range. Kill the encounters with the Sarin/Tronk/Blades icon.

The drop rate IS 10% from normal Raider encounters, 15% from Raider Invasion event encounters, and 75% from the Spiderwasp Queen as a reward (8-15 Skulls). Other ways of getting Skulls are through Boss Strike rewards. Mostly in the lower tiers (easier to get), anywhere from 15 to 50 Skulls.

Hope this helps! I'm always stacked up on Skulls way beyond the maximum. What do you need them for anyway? :)