The easiest way to beat Lupus Boreas in Genshin Impact is by using a combination of Pyro and Electro characters in his Cryo phase and staying close to him in his Anemo phase.
In his Cryo phase, Lupus Boreas will constantly shoot ice projectiles at you if you stay far away. These projectiles deal a significant amount of damage, so it is best to stay close to him to avoid them. Additionally, he will summon ice crystals that form blue circles on the ground, indicating where icicles will drop. Avoid standing in these circles to avoid taking damage.
To quickly defeat Lupus Boreas in his Cryo phase, it is recommended to use Pyro and Electro characters. Pyro characters will be able to deal increased damage to him due to the elemental reaction, while Electro characters can quickly break his shield and deal damage. Utilize character abilities and elemental skills to maximize damage output.
Once Lupus Boreas enters his Anemo phase, he will primarily use range attacks. It is still important to stay close to him even if you are a ranged DPS character. This is because his range attacks hit hard and can be difficult to dodge from a distance. By staying close, you can avoid most of his attacks and deal damage more consistently.
During the Anemo phase, continue to use Pyro and Electro characters to melt his HP. The same strategy applies here as in the Cryo phase, focusing on dealing damage with elemental skills and abilities. Be mindful of any special attacks or abilities he may use during this phase and dodge them accordingly.
In summary, to defeat Lupus Boreas easily, use Pyro and Electro characters in his Cryo phase to maximize damage output. Stay close to him to avoid his range attacks and use elemental skills and abilities to deal damage. In his Anemo phase, continue to stay close and utilize the same Pyro and Electro combination to quickly melt his HP. Avoid the blue circles indicating dropping icicles in his Cryo phase and be mindful of any special attacks in his Anemo phase.