Do you know what Thermal Sunder does, exactly? Simple: It exists to take Heat procs and use them to explode enemies to death forever. And do you know what Blazing Pillage has an unreasonable amount of? Heat procs.

How-to/general gameplay loop:

• Cast Haven(your 3) and keep it active. While the build hinges on having it active, it's not especially relevant - it's really just here to activate Blazing Pillage.

• When near groups of enemies, cast Pillage(your 2) to mark them for shield drain and, more importantly, cause Blazing Pillage to restore a significant chunk of shields and apply a Heat proc to every enemy in range.
  • Then, tap-to-cast Cold Thermal Sunder(your 4) twice, which will, with the first cast, consume all Heat procs on affected enemies for a burst of damage that scales with the Heat procs' damage, pulls all those enemies towards you, and, with the second cast, freezes all those enemies for eight seconds.
  • Then repeat until everything in range is dead.

• You should never hold-to-cast Heat Thermal Sunder, as, while it will increase existing Heat procs, it completely removes Cold procs, which provide free survivability due to their strong slow and the freeze effect from casting Cold onto existing Cold procs.

• Balefire(your 1) also exists, but you should honestly never need to use it with this build.

Why this choice/other options:

• Replacing Aegis Storm with Thermal Sunder - Aegis Storm is cool, but it's not very good. Thermal Sunder synergizes astoundingly well with Blazing Pillage and works as fantastic crowd control.

• Blazing Pillage - Causes incidental Heat procs and restores a large amount of shields. There is no better option.

• Arcanes - Arcane Barrier is extremely helpful for any Hildryn build, but Arcane Aegis can be swapped for a different Arcane of your choice.

Tips and Tricks:

• Thermal Sunder is much, much stronger casting Cold onto existing Heat procs, rather than Heat onto Cold procs, as it removes the opposing element's status for bonus damage based on remaining proc damage... which Cold procs don't have, which accordingly greatly reduces the potential damage.
  • Blazing Pillage accordingly synergizes incredibly hard, because it generates Heat procs to remove by casting Cold onto them... which means you don't need to cast Heat for Heat procs at all, and accordingly are always casting Thermal Sunder for high damage without removing Cold procs from enemies, which means all enemies involved will be frozen.

• Since she has a massive, regenerating 'energy' pool, Hildryn functions wonderfully as a Railjack pilot.