There is much more of human baddies in this game than in the first one. They are quite intelligent and can follow you onto higher places, or simply shoot you. Kill them using side jumping and shooting. Now you'll have two Rexies outside, but also a weapon that can relatively easily blast them. When you see a spiderweb on the wall, you know what follows.
Spiders are quite weak, but still very annoying.
Don't let them surprise you. Flying enemies - expect them on large open areas. Don't let them get too close. As you go towards the end, enemies are getting harder to kill. The best weapon to use on yeties and all sorts of human warriors is the grenade launcher.