Talk:Raider Training Camp/@comment-1838919-20140914072927/@comment-11090687-20140914103453

They drop 100% (3 Skulls each) from Raider encounters with one of the Raider Lieutenants (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! :)