EDIT: This guide is outdated as of 6-21-2017 with the Rise! update. Lavasat's charge mechanic has been removed, and it will drop one extra core for breaking any part.
Don't be a coward.A lot of this info is just re-posted from the old NA forum and the only info I can take credit for is the Flying Snow cancel.
Phase 1:Strategy - Most people just zerg this phase, but if you're having problems, have someone distract Lavasat in the middle, and have the rest focus one Hellspike at a time.
Tip - There is a glitch that allows Karok to skip this phase entirely by using his clash near the red barrier. His clash animation will place him behind the barrier and allow him to begin phase 2, however if someone is attacking a Hellspike, then the HP bar may display the Hellspike instead of Lavasat's for the entire phase 2 fight. To avoid this, Karok can wait until the Hellspike's HP depletes. Video demonstration:
Phase 2:Detailed Info-Most already know this, but Lavasat has 5 armor pieces (one plate on each leg and the tail) that dramatically reduce damage dealt to it, alongside the main break-off on its back. These 5 armor pieces function like ordinary break-off parts and will shed when enough knockdown has been dealt to them.
-When Lavasat glows red, it becomes immune to damage and will charge at one of the lava flows at the edge of the map. If it hits near the lava, it'll regenerate all of its armor. To avoid this, you need to fire the chain hooks provided at its horns and steer it off course. It'll do this approximately every 70-80 seconds on 80-90 mode, 100-110 seconds on 70-79 mode, and 170-180 seconds on Easy mode.
-Either horn can be hooked to steer Lavasat away from the lava and only one person is required to succeed, but having more people usually helps. All party members should be hooking on the same side or else you'll be working against each other, and it does not matter which horn you hook; you will still steer Lavasat towards whichever side you are on. If your chain does not light up when it lands, it means you did not hit the horns and you should cancel your chain to avoid steering conflicts. While hooking, you need to use the back directional key (S) to steer; simply landing the hook is not enough. Also, hooks should be fired at as close to 90 degrees as possible to the horns or steering may fail. Illustrated example:
Strategy - Having a timer that spams "I'm sorry!" (F3) 3 seconds before Lavasat glows red helps a lot. If you've been keeping Lavasat at the edge of the map, it will usually advertise itself by running or jumping in a zigzag towards the center of the map. Note that Lavasat's charge may sometimes get delayed by a hold skill or outright canceled if it's flinched right before turning red. If Lavasat does not glow around the expected time because it might have been canceled, just have confidence in your time-keeping and calculate the next expected charge.
Strategy - If your team tends to split on each side of Lavasat, do not be afraid to call out which side they should focus chaining on, and if things get really desperate, tell your team to leave the chaining to a reliable few.
Tip - Greater Mysterious Cat Statue has a cool-down of 70 seconds, which can be used as a reliable timer for Lavasat's charge mechanic on 80-90 mode.
Tip - Vella's Frostbite order begins with Lavasat's left back leg (1 flinch), left front leg (1 flinch), right back leg (1 flinch), right front leg (1 flinch), the tail (1 flinch), and then finally the main break-off being the horn on its back (5 flinches). The only break-off that drops loot is the horn.
Tip - Fiona and Lynn can interrupt Lavasat's armor-regeneration with a well-timed Shield Charge or 10-stack/Jade Fox Flying Snow respectively. Other skills may work, but the window to interrupt is so short that these skills are the only ones I know of that work (you're practically fishing for it). Video demonstration:
