Necrons are probably the hardest for me out of the three armies you'll be facing.
For IG, just remeber to take a few things to kill tanks. Other than that, you're good. They are weak in CC. You already stated that you know how to fight them so I won't comment.
Daemonhunters are definitely not unstoppable. They're probably, for me, the easiest of the three. Really they're just fancy terminators and marines. Get them in CC and your choppas will ruin their day. Again AT is very important; they probably have a landraider. Rokkit launchers are good because they can destroy any vehicle and they are AP3 so those marines won't get their saves.
For Necrons, speed is the key. Get in cc as soon as possible. If you are going to shoot, try to shoot at one unit at a time. Don't underestimate the lord; kill him if possible because he probably has a res orb. Avoid the monolith. Do not shoot at it; you're better trying to get them to phase. Remember, you can assault more than one unit at a time. With 30 boys you could assault two units of warriors and wipe them. This is another trick against the monolith. If your opponent has to use it to get his necrons out of CC, he won't be able to use some of it's weaponry. Remember, not everything in the necron army is a necron. You don't have to kill the Ctans or the monolith, just the necrons. Find out which ones those are by reading the codex, and just kill those. Making the necrons phase is very enjoyable.