Firing Speed :: for maximum firing speed a Lime and Cyan combo will do nicely
Range/Power :: for power and long range kills a Red and Yellow gem combo will be your best bet, though need a couple of level 4 gems to start a decent combo
Slow :: to slow multiple enemies a Lime and Blue, or a Red and Blue (although the red and blue is a lot more effective and always 100% efficient it takes a long time to have a decent combo)
Mana Pooling :: for a combo thats sure to turn a small pool of mana into an ocean of power and defence (if mass banishing is required) a few traps with Red and Orange gem combos, and some towers with Lime and Orange gem combos will be a blessing. WARNING : The mana pooling techiniques are extremely hard to use since traps dont inflict much damage, and finding the right colored gems is also a challenge, this plan also requires mad skills as its extremely hard to place the right gems at the right time and can be a blessing in desguise by creating the illusion that you are unstoppable, thus leading to missuse of mana like overangering.