The game has many objectives and killing monsters is one that's extremely important. Destroying the correct monsters will gain you some precious EXP while dying will help you lose some.
Even beginners will probably realize that kooks are easiest to kill. Here's a list of the easiest-to-hardest beginner monsters: young kooii, kooii, bramble donguri, leaf boar, angry leaf boar, bulldozer jr, BULLDOZER, BULLDOZER BROTHER.
Below is a list of all the monsters you could encounter. Also feel free to check the Guide to leveling up, monsters needed to level up, and weapons section.