from what little I understand about Immortal Builds, in this case, it looks like this Immortal Build has a lot of defense. for every round after the first, all damage done is increased 25%, so eventually, even though it was doing small amounts of damage, the damage goes up every round, same for players. so if you were able to survive somehow, you would eventually be able to do enough damage to defeat the Immortal Builds defenses.
do a search in the Battle Arena posts. there is a lot of good advice, and some of the players have shared video of their Arena battles.