flamers and screamers (the manta things) are very, very good tools now since the WD update, but they arent unbeatable. Basically DoC have a few huge weaknesses as an army in general, these are.
A: Weak Troops: Yes, I know this sounds funny, but their troops are god aweful. Their best being plaguebearers, but basically they are glorified wracks. Their job is to sit in cover on an objective and wait things out. Them and a few pink horrors are about all you see in a basic DoC list, and normally only 2~4 squads total. Since 5/6 of the basic objective require troops to win, you can easily knock out a DoC list by focusing on their troops, that means they have to contests every one of your objectives to tie.
B: Long Range AT: Yes, this is a huge problem. Their longest range AT is bolt, which is only 24". Now flying MC rules has given them some great manueverability now, but basically they still need to finish off tanks close and personal (whether its flamers with breath or screamers chargins) Keep a distance and your be fine, even if that means T2 turboing away from them to give you more ground between you and them.
C: Deployment is random: Now this can either be a boon or a curse, as it all depends on how many times you play against Daemons. Their weird deployment catches most people who dont play them off guard, and can wreck armies entirely off of T1. After you play them a few times though, your begin to see its biggest weakness, how random it is. Not only do you have to roll to even make sure you get in your prefered wave, but you also have to deepstrike everything!!! Your learn that by grouping up together, you make that DoC player make some hard choices. Does he go sucicidal with his flamers to make sure they are in range, but have a huge chance of scattering and rolling on the deepstrike mishap chart, or does he play it safe, but that means they are out of range for a turn or two. This is something I know every daemon player hates, and you can easily get into their head T1 if they make too many mistakes.