Control.

Seth was at an advantage here. He was smaller which gave him more ground and a better hiding spot. 

As the monster rushed at him, Seth jumped up and landed on its back. He used his sword as a means to support his back and hell did that inflict more pain on the beast.

The beast got crazy as it staggered at random places in the forest but it was not enough to get Seth off him. 

Eliza became curious. Ignoring Seth's decree, she slowly peeped at what was happening. The moment she saw the position Seth was in, she instantly went back into hiding while she covered Agatha's mouth with her palm. 

But that little moment was enough for the monster to recall that there was still a human behind that tree. It figured that going after Eliza might be the only option it had to get Seth off him. 

It ignored the vampire at its back and began to rush back to Eliza and as expected, it was enough to make Seth jump off its back and obstruct its path.