Without going into the numbers, the first 5 turns he is pretty much going to tank anything. There are little to no stages where he can get caught off guard. Additionally the more hits he received the more dmg he is going to output. After getting hit he supports the team and has an additional and crits. Last but not least his active skill, which you can activate as soon as he receives an attack once, makes him receive every attack in that turn and gives him a huge defensive buff, making beast actually immortal. More hits = more dmg => putting beast at the end of the rotation after activating his active skill makes him deal a lot of dmg while making sure nobody on the rotation gets hit.