I am 11 hours in the game with my starter base (blue science tier) when I get a secondary mission which starts with an error message: "unknown key: 'missions.secmission_10_txt'". A giant boss worm with 100k hp spawns on my map and shoots fire at artillery distances at my base, laying waste to it.
"Luckily" I have the Robo Army mod as well, which allows me to bring 20 Terminators (mobile laser turrets) down on it with me throwing grenades and firing as well. I have tried several times now. On my best run I brought it down to 93k before my army was destroyed and it is regenerating. I now have the option to either quit this game and start over without MSI (because there is no killing this thing) or I cheat and disable all achievements. Anyone other ideas?

Sorry, but I'm reverting my previous comment. This is a new side challenge for me. The author must have added it since my last play-through.
Now that I'm at that point in the scenario, I'm with you. How do you even get CLOSE to that thing? It fires on and destroys my pipes as I try to lay out for flame turrets.

Come ON Marco, give the player a friggin' chance, eh? Sure, side quests like this should be challenging, but not so impossible that the player loses interest in continuing the game!.

I changed the damage of normal ammo to 10000000 and did a hit and run attack with a car.

If I hadn't known how to do that, I'd have removed the mod and played on without it - it doesn't seem to make any significant changes to production lines.
Since the code is obfuscated in some obscure language (certainly not english/german/japanese/russian/french/latin), you can't even remove this event in the code.

Hi guys. My group just loves this boss. But If you don't, kill it with this command:
/c local surface=game.player.surface
local e = surface.find_entities_filtered({name="msi-worm-boss-fire-shooter"})
e[1].die()

Hey everyone, fire spitting worm was the most difficult enemy for me (so far), but it was manageable.
Power armor (i usually craft it asap, because of bots anyway), hand flamethrower, some grenades, some turrets as distraction (and help against horde of biters) and loooots of dodging

@MFerrari Since the bosses significantly impact normal gameplay with no option to circumvent them, there should be a mod option to disable them.

Ok. I will make it optional

New version released, with an option do disable this boss (but only if it did not happen yet)

2 nuclear rockets and he was dead...

Yeah, it is too easy now... that is sad =(
I had to nerf if, as many players had complained about it.