Shiva wanted to win in the Colosseum, not just participate but to Win. By now he understood the fact that the harder the challenge the Tower threw at him, the juicier the rewards would be. That was all fine and dandy, but the problem was that there were going to be more eyes watching him within the Colosseum, and there is no way that a massive guild like Bramhalok was going to miss this chance to finish him off for what he did to them.
However there was two problems.
One - A large guild like Bramhalok would have probably find out about the fact that he was the one who killed Dyce.
Second- the fact that he is a necromancer. He would have to face a lot of problems if they were revealed.
The safer path would be for him to not use his summons during the competition, keeping them as a "get out of jail free" card in case something "problematic" rears its ugly head, but to do that he needs to be powerful enough to actually not rely on them.
Forsaken by the Gods, I made a contract with the Demon King