"Human, if you want to live, use all the trump cards you are hiding."
The Ancient White Dragon Emperor was excited to see that Leo was not only unharmed but all his injuries were healed from the attacks of the Legend Grade City Leaders under him.
Therefore, he asked Leo to use all the trump cards while fighting him because the Dragon Emperor also wanted to have an all-out fight with someone before the Inter-World Competition starts.
"Of course."
If it were before, Leo would not have had enough confidence to fight the Dragon Emperor; however, with the changes in the [Nether Rage Booster] and along with the other trump cards under his sleeve, Leo was confident in defeating the [World Emperor].
"If I start first, you might not have the chance to use all your trump cards. Why don't you attack me first?"
In the Dragon Emperor's eyes, Leo didn't have the ability to defeat him, hence, he gave the chance for Leo to attack him first.