After a second tutorial in which you'll learn how to use weapons, you'll find yourself in a small village area on the other side of Halstedom's outer gate. As you might expect, the developers placed the gun tutorial here for a reason - there are weapons located in this village you can use. The fish-guns are located to the rear right near the center buildings, while a skullbomb gun is to your left. I prefer to pick up the fish-guns. As Daedra warns, there is trouble coming.
Looking towards the gate, you'll see strange creatures leaping through the holes. I can't honestly tell you what they're called - Daedra seems to call them "bumkrubs" - but they are four-legged hopping creatures that like to bite you if you wander too close. The fish-guns can take out these guys quickly however, and are accurate at long ranges so you shouldn't need to worry much about being hit.
Once the "bumkrubs" are all dead, a brief sequence will play. Then you will be in another three-person fight, this time against Gastronis, Xetes, and Rimat. Xetes seems to be a fairly aggressive fighter who will sometimes charge attack you, but Rimat is still the most difficult. You'll be working with a new dynamic in this fight, since guns are involved. If you stay towards the rear of the map you may be able to knock off one of your opponents with the fish-guns before melee starts. Once melee does start however, I suggest throwing the fish-guns to the floor (press space) and focusing on your brawling skills.
Because you have so much more space in this battle, the actual melee combat should be easier than the first fight. But be careful, because opponents can use guns as well. If you drop a weapon they will pick it up and use it again you. If you seen an enemy pick up a gun, charge over and give them a whack. They'll drop the guns, and most times they'll be momentarily vulnerable. Once you've vanquished all three of your siblings, this chapter will end. Don't worry about having a gun on you, because it won't be available at the beginning of the next chapter.
This is the end of the Prologue chapter and the end of this portion of the Walkthrough. The next section of the Walkthrough will cover the journey away from Halestedom and towards the forest of the Coriwids.