Ya the job looks complicated from first impression but it is actually one of the most simple Melee jobs in my opinion.
Viper basically only has to focus on using a 1234 Combo with there being 2 types for both single target and AoE.
Normal Combo always starts with two skills on Combo 1 which is either a raw damage attack or a Debuff attack that makes the enemy take +10% more damage.
Then skill 2 is just use any of the two Combo 2 skills. The order does not matter since the one you did not pick will be your next Combo 2 skill.
Skill 3 is the one players need to pay attention on because the color of Combo 3 skill determines their position for flank or rear. Red skills are Rear attack skills and Green skills are Flank attack skills.
Then end the combo with the Finisher skill being the skill that is Serpent's Tail.
----------------------------------------
Burst skill is the same but the position attack is on Combo 2 skill
Just use the starting Combo 1 skill then use one of the two Combo 2 skills being the Flank or Rear attack skill.
Depending on which Combo 2 skill you use for Burst combo determines which skill will be used first between Twinfang and Twinblood. Then just use the other Combo 2 skill you did not use to finish off the Burst combo chain.
After that it is just waiting until the Burst skill combo cooldown is over by using Normal combo then use Burst Combo again.