He activated the call for support because he believed that he was safe to act within this world fragment. It is not a baseless belief. He has the Oath of the first Sage to reassure him. He also can't die permanently in this competition, so he should be safe.
He has had many threats, and he has been afraid because of them. Lady Amari is just one of his enemies. She may be small and insignificant, and her death here may be temporary, but he will allow himself to enjoy it. What is life without the little pleasures?
Now that it has been decided that he will crush her. It is time to decide how he will crush her. He can do it on his own. He has the power to. He hasn't used the call for support much yet, but what he already has is enough for him right now. The call for support makes him do what he can do, but he does it better and more powerfully. He was already more powerful than his enemies. Now that his shackles have been removed, he outclasses them by miles.