Try to be judicious with the use of your gems. If you’re the kind who likes to speed up your game by blowing gems, you’ll save a TON of them if you train monsters that use a lot of housing space. Every monster that you speed up with a gem costs exactly ONE gem, so if you speed up 25 Skeletons, you use 25 gems, while if you speed up just 5 Bile Demons instead, you’ll spend a mere 5 gems.
Before you do ANYTHING in your dungeon, hit the “plus” sign next to your imps, and then when the imp management screen pops up, swipe across and slap your imps. Every time that you slap your imps in the face, they will build and upgrade anything you want to at twice the speed for half an hour. Keep re-upping their 2X boost for long jobs or for digging up gem veins.
Try to send in as few troops as possible, because not only do troops cost gold, but as mentioned above, they cost gems as well if you decide to speed them up. Often, you can beat campaigns fairly easily with less troops, especially defensive campaigns if you have all of your available traps built and if your pathways are twisted enough to severely slow down the attackers.
Spells are important, too, especially for the defensive campaign missions. You can beat entire waves simply by turning a group of enemies into chickens and getting rid of them. Keep upgrading your Dark Library to get stronger spells.
The same is true for when you are attacking other players. A good trick for this is to send in a few trolls first to distract the traps and defensive rooms, and then send in a bile demon to go after the treasures and the stone storages. You might not finish with a very high score, but you’ll finish with a large load of resources.
This serves multiple purposes, as well. If you get too many trophies, you will end up getting matched up against tougher players when you go to the “find opponent” mode, but when you lose you will lose trophies, and thus get matched up against easier players. Send in a couple bile demons to destroy the warehouses, then end the raid before you get 50 percent, and you’ll take a loss, but gain resources, and open yourself up to easier values later.