Silas, of course knew that Elizabeth was threatening him but he wasn't worried. He was completely aware of his strengths and he already made an estimation on what a regular Phenomenal's strength is.
Due to their inability to absorb dark energy, every Phenomenals on Earth, with the exception of Silas, have their superpower unranked and the only way for them to increase it is to take the GES.
Silas' confidence stems from this information and the fact that he has more than enough strength to bring down the underground arena, and still walk out of here without a scratch.
Silas looked at Elizabeth who was looking at him with an icy look in her eyes, and he smiled. He seemed his previous judgement on her not wanting trouble was wrong. Maybe he will get his chance today.
"Am I confident? Maybe.. Maybe not," Silas replied with a smile, and stood up with the intention to leave.