He can't target your Koth with Valakut even if you don't have the Leyline, because Koth is neither a player nor a creature.
That said, he can't HIT Koth if you have Leyline. You can't be targetted by your opponent's stuff, so they can't target you with the Valakut and have it damage Koth on resolution. The Redirection rule is simple: Whenever noncombat damage would be dealt to your opponent by a source you control, you may have all of that damage dealt to ONE of your opponent's creatures instead. So, for him to burn your Koth, he'd have to legally target you first, which he cannot.
Planeswalkers get dealt non-combat damage through redirection of damage that would be dealt to the player that controls them. In this case you are not a legal target for Valakut's trigger so that isn't possible.
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That said, he can't HIT Koth if you have Leyline. You can't be targetted by your opponent's stuff, so they can't target you with the Valakut and have it damage Koth on resolution. The Redirection rule is simple: Whenever noncombat damage would be dealt to your opponent by a source you control, you may have all of that damage dealt to ONE of your opponent's creatures instead. So, for him to burn your Koth, he'd have to legally target you first, which he cannot.
The latest Comprehensive Rules are also good, and can be found here.
Planeswalkers get dealt non-combat damage through redirection of damage that would be dealt to the player that controls them. In this case you are not a legal target for Valakut's trigger so that isn't possible.
Ever wanted to know what guidelines Judges use to make rulings? Find out at the DCI Document Center.
You can also find the latest Comprehensive Rules here.
Trades