Your chosen character will have their respective battle modes or also known as BMs. It differs per each class and in Cabal Poison, this is attained easily by completing a few quests.
There are three levels of BM. BM1 and BM2 are automatically set on your character. You can go to your skills by pressing "K" (this is by default), then go to the "Special" tab. Under the label Battle Mode, you will see the description of each BM your character has. You can drag the icons towards your quick slots to easily activate them or you can right click on it.
BM3 is your character's ultimate battle form that can be attained. In order for your character to attain BM3, you must need to attain Battle Style Level 14 and character level 140. Once you are complete with this requirements, you need to go to Port Lux and look for "Chloe".
You will find all the necessary skill books related to BM3 on that NPC.
As for the Synergy (combo for BM3) you can use the below formula or create the one you want:
AABA + BAAB + ABAA
Here's the table I found online for BM3 Synergy (Credits to Cabal Online):
Subject: Battle Mode Guide 
Thu Aug 04, 2016 4:25 pm