Elements

There are four elements in Dragon Project: Fire, Water, Earth and Lightning.

Every Behemoth with an element has element weakness. There is also behemoths without an element! (e.g. 

You can check their element and weaknesses on the Quest screen before you start the battle

Elemental Attack

If you are going against an Earth-element behemoth, you should equip yourself with a Fire-element weapon to increase your damage! (You will be surprised that there's actually a huge difference!) Oh and you should attach a Fire attack magi in your Fire-element weapon too!

*Not every equipment has elements. Also, you can sometimes choose between a normal (non-elemental) evolve or an elemental evolve after you have max upgraded your weapon. If you pick the elemental evolve, your equipment will gain an element. (e.g. Wukong Spear)*

Elemental Defense

For Elemental Defense, you should wear an armor that will help defend against the behemoth's Earth Attacks. So if you are battling an Earth-element behemoth, make sure you are equipped with a Earth-element Armor!

*For older Behemoth equipment sets (before Paradise Houran sometime in October 2017), the elemental defense of the armor matches the elemental attack of the weapon. That means if you have an older equipment set such as the Scorcher’s Set, the weapon deals fire elemental ATK and the armor defends against fire attacks.

For the newer Behemoth equipment sets, this has been tweaked so that the armor will be complementary. For instance, the Blackfire Set weapon deals fire ATK while the armor defends against Earth DMG.*

In a Nutshell, 

 Against a Fire Behemoth, Use Water Weapon and Magi and Fire Armor

 Against a Water Behemoth, Use Lightning Weapon and Magi and Water Armor

 Against a Lightning Behemoth, Use Earth Weapon and Magi and Lightning Armor

Against a Earth Behemoth, Use Fire Weapon and Magi and Earth Armor