@DuboBrew_PSN - You need 4x the amount of fans their stronghold has. Attacking 24 with 99 should have had a Rookie option (24x4=96). But attacking the 27 would only offer Veteran and higher for difficulty.
Try your best not to let them get to 25 if you're aiming for Rookie. When they're above 25, I often have to surround their stronghold with hexagons of 10+ each. Then inevitably on the CPU's turn they will try to attack your hexagons surrounding them to lessen your numbers. But it always brings down their number too. If they attack those enough, you can eventually attack with a Rookie option when their number falls below 25. (Assuming a 99 attack)