MArcomage

This is quite an interesting idea

Very unclear card text

It's simple, if your opponent's highest resource is recruits, then you replace all recruit-costing cards in game with cards that use up a specific resource, which you choose by mode

Re-reading kicks ass.
Text is still rather lengthy, though. You could make it appear shorter by removing the quick part. The card doesn't really need that to be interesting.

Quick.
Replace all cards that have cost requiring highest resource type of opponent with random commons of chosen cost type:
Mode1: bricks
Mode2: gems
Mode3: recruits

Let me have a shot at improving the text...

Quick.
Discard all cards that cost the opponent's highest resource. Replace with random Commons that cost:
Mode1: bricks
Mode2: gems
Mode3: recruits

This does little for the length, but I think moving the word "replace" and describing the discard and replacement as separate clauses makes the entire thing easier to follow.

Wording now edited to reflect DPsycho's rewrite.

I like quick because very few mass discard cards have the quick keyword, so I feel this makes it stand out in yet another way, allowing you to immediately use the commons you draw. But you're right in that removing the keyword would free up even more text. /c: