in easy horde: the spiders take one shot, the crawling zombies take 5, the walking zombies take 2 or 3 (go for head shots!) the ones with the balloons take 6,and the big guys that puke on you take 10.
the spiders are really fast, get them out of the way before they jump on you. back up when shooting the big guys, so you don't get thrown up on. the crawling zombies are fairly slow, but cause a lot of damage clawing at you if they get too close. watch out for the balloon guys up ahead too, they can be pretty annoying throwing things at you. they do a fair amount of damage.
when you are reloading back away from the enemy closest to you to buy more time. other for that reason, don't bother moving the guy around. just keep him stationary and let them come to you, trust me they will!
sometimes if you're lucky and shoot a dead even shot, you will inflict damage not only on the closest enemy, but the enemies behind!