Timing, Aiming, Speed and Placement of your mage on the map are all important things to consider
Blast is a Basic AOE attack that can be Very Effective against units of all kinds
Tip #1: You can get the maximum range of all spells by standing on top of the map
Tip #2: Spells can be casted without needing to walk to target when you aim 50% of it inside the lines of range
Tip #3: Magikill can sometimes dodge attacks (Even a Giant's Boulder) by "Side-Steping" upward
Tip #4: USE HOTKEYS!!! Q-Blast W-Poison Spray E-Electric Wall, Especially when you have multiple Mages selected (Tip 1 & 2 shown in pic below)

Poison is a directional attack and it is recommended that you cast it as a Line(Diagonal) + The Spell is actually casted above the aimer(Oval).
Ex.

Electric is a AOE Wall-Styled attack that can cover a good line of the BattleField
I recommend to use it on enemy walls and archers or for defense.
Electric Tip: The electric wall is good for defense sometimes as it forces enemies to retreat.
Ex. If a spearton charges your mage it is actually recommended to cast the electric wall on the center of your mage as this will defend it from both sides

Some Pics for how you should aim for flying units:



Mage vs. Spear battle starring KrayZStick:www.stickempires.com/play?replay=replay923617&version=1.59
Mage vs. Spear battle starring GamerXD8:www.stickempires.com/play?replay=replay923804&version=1.59
Mage vs. Spear battle starring THEFORCE:www.stickempires.com/play?replay=replay1300713&version=1.62 (Error :( )
Possibly more to be added soon and if you think you have some tips I did not mention you can reply :D