@G.S.  
Using a spell like once or twice defensively doesnt mean she’s a master fo defensive spells.
 
Neither defensive nor offensive magic are really her strength, twilight is more utility.  
and just raw magical strength.
 
Also yes he doesnt know shining but, he knows twilight, and twilight knows shining meaning he has a connection to shining over twily.  
So its definitely not farfetched that twi would have shining teach him about defensive spells.

@Transparentist
 

 
I genuinely can’t tell if you’re being sarcastic or not
 
Shining Armor would probably do better at the teaching part, but Sunburst has never met him, and Twilight would be better at guided study of existing spellbooks

@G.S.  
>Sunburst has studied defensive spells under Princess Twilight
 
Because twilight is well known for her defensive spells

@DK2K1
 
Starlight:  
Low HP  
High MP  
High POW  
High SPEED  
Low DEF  
Low HORN ^{%(same purpose as Stache in the original game)%}
 
Starlight typically moves first and hits hard. She hopes to defeat enemies quickly before they can overwhelm her. Her spells are powerful and designed to do as much damage as possible, though her defensive abilities are lacking.
 
Sunburst:  
High HP  
Medium MP  
Low POW  
High DEF  
Low SPD  
High HORN
 
Sunburst has studied defensive spells under Princess Twilight to fill in the gaps in Starlight’s fighting style. Due to the numerous and complex defensive wards he casts on himself, he will often still be standing after an attack that knocks down Starlight, and can revive her with a Healing Crystal.

Starlight and Sunburst: Superstar Saga

@Background Pony #B386  
Oh i thought they were on Horse Tranqulizers

@Transparentist  

 
They have no souls

Why are your pupils gigantic?