Spells in Treasure of the Rudras are called mantras. Creating a mantra with the elemental name by itself creates the weakest mantra possible for that element. Adding a suffix makes it stronger and consumes more MP. There are lots of suffixes in the game. Other Treasure of the Rudras fans might beg to differ, but I believe the best suffixes are ~na, ~rex, ~nates, and ~god.

The following table lists all the basic elements with the suffixes.

MP costs may slightly vary with certain mantras or with other characters.
You shouldn't have all of these mantras in your mantra list. Just use some at a time. At the beginning of the game, I wouldn't recommend using the strongest mantras because they would just consume all your MP in a couple of rounds. You should gradually switch to stronger mantras as you progress through the game.
I left out Lef (healing) because it doesn't work too well with these suffixes. Instead, I added it in the next section below.
1 Soagod creates a Dark mantra, not a Light mantra. Not all mantra suffixes give you the desired outcome.
2 Prana creates a Void mantra, not an Earth mantra.