The Viper Ruler, AKA Viper King, is one of three alien rulers encountered in the Alien Hunters DLC. It is arguably the easiest Alien Ruler to defeat.
It will be the first alien ruler you encounter if you´ve enabled the mission "Alien Nest" at the beginning of your playthrough. If you have not enabled the mission, which alien ruler you encounter first will be random.
Like all other alien rulers, the Viper King performs one action after every action made by XCOM during a special Ruler Reaction phase. Some abilities do not trigger Ruler Reaction, like Lightning Hands or Bladestorm.
At some point, when enough damage has been dealt to it, it will open a portal and attempt to escape (if you kill it as it attempts to escape, you will get the achievement "Not throwing away my shot" if you don´t have it already). If it escapes succesfully you will randomly encounter it in a future mission, but it's health will be the same as last time, it does not regenerate.
It is unlikely you´ll be able to defeat it during your first encounter, however, if you manage to do it, you will get the achievement "Regicide" if you don´t already have it.
Here is the strategy for fighting the Viper King:
He has four attacks:
1. Bind and Crush: grabs a squadmate, deals damage and has a 50% chance to render unconscious.
2. Tongue Pull: same as Bind and Crushed, but ranged.
3. Frostbite: AoE ranged attack that freezes units for two turns.
4. Bolt Caster: shoots you with his bolt caster.
General tips:
-Take off its armor ASAP.
-Hit him with effects that do damage over time, like burning or poison. It will take damage in every reaction phase.
-You can counter Ruler Reaction with flashbangs, frost bombs or attacks which can stun.
-Try to fight it when it's alone. If possible kill all other nearby enemies first.
-Do not group your squad too close together, to prevent multiple of them getting frozen.
-Move as little as possible. Try to use all your actions to attack.
-Use free actions as much as you can.
-You can use the
button to end your turn manually and get back actions from soldiers who already used their actions.
-Psi Op's Stasis ability will lock the Viper King down for an entire turn.
-If War of the Chosen is installed, the ability Dual Strike, available when two soldiers have a level 3 bond, can be extremely useful.