Turning invisible is not a problem. We did use an official agreed to dice roller site. Cyotcode or Invisiblle castle. Not sure if either is still operational. I will check into it.
The bow of this ship is maintaining a consistent distance and aspect in relation to the black ship. Which should be impossible. Both ships are in motion.
You estimate the number of hostiles to be about a half dozen. Two or three of them seem to be acting as shields for the ones with bows. They don't seem to do anything except lumber and take hits from crossbows.
That spell you cast should have requisites for Animal the design. Casting Requisites have to be part of the design. You can't add them as an afterthought. It should be a magnitude higher, but you could have designed it a magnitude lower and flex-cast it. Fixer used to do this with Wirth all the time. Spells that make no sense as Target Part, intended to be Flexed to Individual or Group.
But anyway, moving forward...
That spell seems to be your best option. But since itwas the third thing you thought of, it is fair to say that the hostiles on the back ship get a few shots in. One of the re-loaders takes a hit.
So does The Woad
Archer Characteristic + Ability + Weapon = 7, 1D10 = [9], -3 for Invisible, Total = 13
You were visible just a moment ago and have not moved. That was a shot in your general direction.
Make a Dodge roll, and if unsuccessful, the Damage is 6 + (13 minus your dodge).
That will require a Concentration roll if you still want to cast your Perdo spell.
OR...
You could choose not to dodge, and if your "stone skin" can handle it (damage +19), then you do not need a concentration roll.