Run to the other side at the start, because the opening is not a good place to be. Find the area with two circles of graves. Use those for cover from his gunshots. I used the Hunter Axe, so I don't know what other weapons can do.
Wait for him to attack. Memorize his attack patterns between the two weapon stances. Right as an attack ends, hit forward and R2 to do a leaping slash while in 2H stance. This has massive range and stuns. R1 after that for a second free hit. Then dash backward. Rinse and repeat. You will take some gunshots, and you just need to accept it and heal through them. I never died on form 1.
Form 2 is much more difficult. If you can parry, great, but I suck at partying. Instead, side step a LOT. Sometimes you need to sidestep through 3-4 attacks in a row. Then get a swing or two in. Because he is fast and his health is low, your instinct is to rush him down. Resist that urge and play it smart. Sidestep, hit, etc. Don't overdo it on attacks, you can't lock him down with stuns.
It took me four tries to beat him, and all three deaths were trying to figure out form 2. If you get hit by form 2, you are almost certainly screwed too much to heal. He just moves too fast. I did get two heals off, but it almost got me killed even trying.