Geevra

(I'm open to other systems, but this is my own quick system to keep things quick and fair.)
Each spellcaster starts with 10 Stamina and will secretly craft their spell. Spells are made up of a few elements.

Name/Effect: The name/intended effect of the spell.
Power: The strength of the spell, if two spells go head to head this will determine which wins.
Control: How likely the spell is to not fizzle or go out of control.
Complexity: How complex the spell is, this will determine which spell will hit first. It may even let another to cast another spell if the complexity difference is great!

Each spellcaster starts with 5 points which they may assign to power or control. They may gain more points by increasing complexity (1 for 1)

Both spellcasters will then post their spells and roll an eight sided die..
If control is less than the number rolled the spell fizzles. If its 3 or more it backfires and deals 1 stamina damage to the caster! Else it is cast.
If a spell has less complexity than the other than it goes off first (if this renders the opponent's stamina to zero or below, their spell will not be cast) for every 3 points under the other spells complexity the spellcaster may cast an additional spell of 1 less complexity than their current spell if they wish!

First spellcaster to reach 0 Stamina loses!

If both players agree there is a significant skill difference between the two casters, the more skilled caster can be awarded a number of re-rolls before the match begins, to emphasize this difference in skill.

A player may choose to spend their stamina to increase control on a failed roll. Increasing it be 1 for every stamina spent.

A player may cast a defensive spell, in which case the power cancels out the power of the chosen spell.