On my 2nd single player play through on DI, first time was real hard, but once you figure out your tactics it gets easier, I didn't struggle much on my first run until I hit the prison, Ram + Thug + 5 zombies in a narrow hallway is a death trap. Here are my methods for the 'mini-boss' guys (thugs/ram etc)
Thug: Weave in and out quickly, watch him throw a punch then go in for your attack, try use bladed weapons rather than blunt ones, time your swings and try remove his arms.
Ram: Get his attention at distance, wait for him to run and then dodge, get behind and get a good 2-3 hits with a nice forceful weapon, rinse and repeat, if your in the town, get his attention and jump on top of a car, you wont take any damage on a car, and strike down.
Floater: Grab his attention, let him spit at you, dodge it, run + kick then get behind him and finish him off, remember floater acid is flammable!
Suicider: Keep your distance, if you have the ammo, 2-3 well placed shots should see him explode, taking out ever other zombie in close proximity. If you have no ammo just throw a few weapons, you can always retrieve them once its dead.
Butcher: If you can take them out at distance, explosives or gun shot. If you get up close and personal try catch them out, if they don't see you coming a kick to the back followed by a few swift blows takes them out, remember to use a weapon you can swing quickly (knife/mace). Also if they are alert don't try kick them, they will dodge and counter.
As for the rest of the infected as said above, just kick them away and focus on one or two at a time, if they have green steam coming off them (floater acid) they will burst into flames and die with one hit from a fire based weapon.
Deo-bombs are very effective against anything, see the trader in the church, you can usually get deo-bomb parts for ~$300 (2 tins of deo and 1 duct tape)
Take your time, enjoy the game and theres no shame in running away if its getting too much