"Before you fight worthy foes, check in with Ghrasharog to prepare yourself. He avoids Cyrodiil or Battlegrounds, which he swears made him complacent after winning victory upon victory."
Worthy foes. The only worthy foes I can imagine are dungeon and trial bosses. If it doesn't work, the tooltip is kinda shady, cus it leads to misinterpretation.
Note that there are some restrictions as to when and where you can activate Ghrasharog (such as in the middle of a Dungeon or Trial run), so be sure to summon him before you begin your activities.