Get all 40 gold first, and don’t wake up the guardians before you attack the crystal. The only thing that wakes the guardians is breaking crystals near them, so take the long way round them by mining through walls. Then they won’t join the crystal fight until the crystal is half dead because they have to walk there.

During the fight, prioritize breaking enough crystals so you don’t get trapped by the giant crystal’s attacks. That attack also hurts guardians, which is fun. If you get surrounded by guardians, do a big loop around the caves until they get stuck in narrow passages. If it’s only one guardian that got close to you, just murder it. They’re not that hard to take down. Obviously, they only die temporarily, but you get a lot of turns to hit the crystal when the guardian is recovering.

If you let crystals grow on one half of the fight area, sometimes guardians can get trapped there and you only have to worry about one of them. Maybe not completely trapped, but certainly slowed.

When the crystal is near death, stop worrying about the guardians and just kill the crystal because doing so kills all the guardians.