This is the most difficult achievement in the game. The requirements for 5 skulls is to never lose a rift point, meaning not only can any enemies sneak through but you can never die, and it must be done in a certain time limit. The time limit should not be a problem, although you may cut it close in the earlier levels, but it is important to keep in mind.
Being a strategy game, there are of course many ways to handling any given situation, but these are some general tips I used to help me make way through the campaign and getting all 5 skulls.
-Always block off split paths whenever possible so that all the enemies will be forced to file down whatever path you have lined with traps and so that they are easier to watch and manage.
-Combine effective traps, namely the tar trap and any devastating wall or ceiling trap. In particular, the tar trap with a clockwork mace swinging over it is extremely effective in any tight choke point.
-If the enemies will be coming at you from anywhere near an opening in the floor standing there with the wind belt blowing them into the abyss is also very effective (and satisfying). This will also earn you bonus points for the kill streaks. Again, lining the area with tar traps to bunch everyone up will help immensely.
-If you are having trouble with a level, don't be afraid to try something new and re-assess the map. Maybe a different trap or way of attack will help.
-Don't spread yourself too thin. If there's a level with two directly opposite lanes and you cannot possibly watch both, place every trap at one lane, hope it will kill or at least slow down everyone, and watch the unprotected lane yourself. Similarly, if you're being attacked both on ground and from the sky, set up guardians and traps to exclusively deal with one problem while you deal with the other. In both cases, you should be watching the one with more enemies or shorter time to reach the rift.
-Give every trap a try. Everyone has their favorites and what they like to use best, so give them all a shot.
-Standing next to the rift will regenerate magic faster and also bring back health. Use this if you think you're going to die and there's no health potions around, or it's unreachable. Also use this if there's only one rift, as you can often just stand next to it and protect it semi-effectively with your quickly-regenerating magic.
-The ice amulet takes out the flying enemies very well (one shot kill and easy to hit) and the secondary attack on the ring of lightning (the lightning cloud) are good for crowds and specifically kobold runners, or if you have to run to another area and want a certain place covered.
The only specific level I can provide advice on is the finale, as none of the guides out there were working for me and I used my own method and got it first try. Set up each entry point with a clockwork mace and three tar traps underneath. Once you have that setup, purchase knowledge weavers to move faster and give the rift it's own defenses, and then start placing paladins at each entrance to defend the rift. Just stay near the rift and keep in constant movement and you'll be fine. I got 5 skulls on War Mage and beat it in Nightmare on my first try using this method.
If you are having trouble with a certain level, check the guides at xblafans.com or check YouTube for countless other videos. There's also a useful thread here that covers most of the levels.