For the first few levels (read: first level and two on each path), spamming LV1 gem bombs (I sometimes call this "gemocide" for the red puffs when that gem goes off) will deal with a great deal of enemies. It also gives large numbers of points and mana, surprisingly. You kill faster, so you get to send waves sooner, so more points. Sending sooner accelerates mana gain in that time.
Translation: Early on, spam LV1 gem bombs. If you get Gem Bombs Upgrade as a skill they hurt A LOT!