When trying to destroy Starro's Starrophytes, what are the rules for how they can be destroyed? Can they only be destroyed per the rules for destroying terrain, or could you destroy them with 2 or less damage/pulse wave?

They are walls, dude.

You need 3 damage, in his rules they are considered terrian for destroying purposes.

My understanding is that they don't block LOF to or from Starro. So in that regard they aren't walls.

Right but for the Purpose of destroying them they are considering blocking which cost 3 dmg to destroy, TIP For STARRO ppl with Flurry and 3 dmg, and someone with 5 dmg with two lighting blots and someone with perplex to get it to 6 to split it up to two 3s

They don't block line of fire to Starro because he is a Collossal.
They do block line of fire between grounded figures. So they could
block for purposes of PC or some such.