Hey, hey ... Just had a battle in Demon invasion.
This image shows that the rage meter is filled and ready to double your scores. When I logged in into the game, I was immediately directed to Demon invasion. Morale was 100 and the rage meter just needed one more bar. So I hit once (that was 25 morale) and the rage meter was full again. Continue to attack until all rage is used up. Usually after all 75 morale are used up, there would still be one more rage. Mine has always wasted because need to wait until morale filled to 20 then I use my morale voucher but I usually forget about it. RAGE ONLY LAST FOR 20 MINUTES. REMEMBER that.
Lucky me in top 10.