Where to find Suicune?
The quest begins in Cianwood City where you will have to talk to Eusine, he can be found directly in the north of the Pokemon Center. Be sure to know about Berry tricking before catching it to ensure you get a nature you want!
First you need to reach Cianwood City and talk to Eusine, Eusine will challenge you to a battle which you must win to begin the quest, his team will consist of Drowzee, Haunter and Electrode.
When defeated, you will then be able to find Eusine on Route 42 between the two bodies of water, slightly south of Hiker Benjamin.
You will need to talk to Eusine again which will lead to an interaction with Suicune.
You will then get a new side quest; Legendary Suicune which will direct you to the Dragon’s Shrine in Blackthorn City. The quest continues upon entering the Dragon Shrine (this will require the HM Surf and Whirlpool), your quest will update to “Read the lines written on the pillars (0/4)”. This can be completed by interacting with the first four pillars inside the Dragon Shrine.
After doing this, your quest updates again, directing you to “Meet Eusine in Mt.Mortar”. The easiest route to find Eusine in Mt.Mortar is by using the entrance found on Route 42, north of where you talked to him earlier.
To reach Eusine inside Mt. Mortar follow the map below (black arrows mark the easiest path). You will need HM Surf and Waterfall and to have obtained the 8th Gym Badge in the Johto region.
When you reach Eusine he will challenge you to another battle, his team consists of Hypno, Gengar, Electrode, Gengar, Hypno between levels 45-50.
Once defeated Suicune spawns and you can finally catch it. This will also cause your quest to update to “Challenge Suicune” which you can now do by simply interacting with it.