Mayhaps:
    ...That player must choose and discard 2 cards, plus 2 additional cards for each of Grievous's Magnaguards still in play.

(And though professional opinions vary, I will to my dying day contend that a singular noun ending in s still receives an apostrophe + s to indicate the possessive.)

The Sultan

You're only young once, but you can be immature forever.
Dorkistan