The Cow Mangler is quite frankly a lost cause. You could maybe allow it to hold more rocketlazars in it's clip, or maybe have a faster reload. Since it can't be crit boosted, it just wouldn't be as effective sadly.
Moving onto the Bison, I think full crits could work. There's very little knockback to the Bison so it won't knock around Hale so much. Also, if you're across the map and you manage to hit him with it, you deserve that damage output.
Pomson could drain a single percent of Hale's rage maybe, giving the Engie some sort of support shotgun.
Eureka Effect.. No idea on how to improve that.
Phlog could increase durability during the rage period to survive two hits maybe.
Third Degree?