1 Get behind it
Most of the Cleric Beast's attacks reach the front and the side. If you always try to stay behind it then the chance of getting a hit will become low.
2 Attack from behind
Keep dodging until you get behind the beast then attack fast. When the beast turns to face you, dodge again until you become behind it.
3 Molotov cocktail
The Cleric Beast is vulnerable to Molotov cocktail. Using some during the battle will help you lower its health.
4 Don't keep a large distance
When you move away from the Cleric Beast it will start doing its long range attacks including jumping and landing on you. By staying close and behind it, you can reduce the chances of those attacks.
5 Level up first
Since you start the game with a pretty weak defense and low HP, leveling up before fighting the Cleric Beast can make the fight much easier.
6 Store blood vials
Since this is a tough battle, you might need some blood vials. When you collect enough of them head towards the boss.
7 Escape all other enemies
When you are ready to fight the boss, you can run away from all of the enemies starting from the lamp and up till the Cleric Beast in a few minutes. You don't have to keep killing all the enemies every time to reach the Cleric Beast.
8 Stay away when it's enraged
The Cleric beast will get enraged after it loses some of its health. Stay away from it until it calms down again then come close to it and repeat the same steps.