There are ways to deal with the Armor Mode, which is that constant Rage Mode phase(where he actually takes .10 damage instead of .20. That's half damage from melee areas, but a whole 1/4 of the damage that the face would take. It's a pretty nasty phase):
1. Bombs deal fixed damage, so one could bring bombs and combines for more bombs. Alchemy Guide would be basically required, allowing for more than 25 Large Barrel Bombs. With Felyne Gunpowder, you're looking at even more damage that Fatalis can't reduce.
2. Shelling from a Gunlance also deals fixed damage, which Fatalis can't reduce. Normally, that means shelling royally sucks, but here, it makes shelling useful.
3. An expansion of bombs, there's the suicide cluster method, which calls for a Bowgun user spamming all the Cluster shots and bombs that they can as fast as possible, then dying three times. On quests where you can repel the monster and fight it later with the same damage, the damage you deal before you fail carries over. I doubt this works in the Event Quest, where the only options are to kill Fatalis in 50 minutes or fail.