Anyone Play Horde Alone?

I pretty much stick to the shotgun and I almost never use it on the wretches. A single melee attack is good enough to drop them. Since I'm playing alone I never mess with the sniper rifle. I also try not to spend too much time angling for a shot with any of the rifles. I spray and switch back to the shotgun. I always play as Cole, his one liners when taking down Locust or just picking up ammo are funny.

"I don't need no crosshairs to kill you" - killing with a melee attack

"My momma could kill your weak a$$" - killing a wretch with a melee attack

"Oh wee save that for the highlight reel" close quarters headshot with the shotgun

"Mines now", "Lookee here" - picking up other guns or ammo.

im up to 18 on my own and its getting very hard

good trick though. kill the big locust that carry the sheilds (i forget their name) then take the sheild and stick it in the ground in a narrow road or ramp.
this stops the big locust getting past and sneeking up behind you

People will look and say 'That poor disabled person CAN FLY!'

I beat all 50 on normal on gridlock. Just grab one of the boomshields and set it up on one of the side towers where the weapons spawn. You can close up the only entrance with one boomshield and then just pick them off. If someone tries to send a boomshot your way, just duck behind cover and it will go over your head. The only tricky part is you have to make sure to take out all of the smaller locust or they could kick the boomshield down.