All three missions came with their own set of difficulties. While the first one was something at an unknown scale, it should be extremely difficult to solve as it was a System mission to procure a knowledge crystal.
Meanwhile, the mission 'Kill the King' had a difficulty level which allowed Shane to have some preparations in place.
Even though he was not sure how strong the three headed blazing lion was, it wouldn't be an exaggeration to say that it was an existence on the peak of the basic primordial dimensions.
It may have not participated in the Primordial tournament, but to some beings, this was not important either way!
Making assumptions was helpful because Shane estimated that the three headed blazing lion was similar to the king of an entire dimension if it was labeled as King of the Savannah.
In the end, Shane's biggest worry was the last mission because he would have to enter the ocean which would restrict his movements to a great extent!