A few hints on beating some of these.
Leon Bell; ride after Leon then get near the water feature, get on foot and wait for him to ride at you. Before he hits you jump up onto the ledge and he'll bump into the ledge. Jump down and attack him. Do this a few times and he'll eventually die.
Brandon Whittaker; the LMG is your friend. Have this equipped before you enter this fight. Then just shoot at him until he's a couple of paces away from you at which point you should jump out of the way. He'll lunge at you but miss if you time it right, but he'll still be crouched down having missed you so get back to firing. Do this for 3 to 4 goes and he'll be close to death. Just finish him with blade gloves if you have them. Using this method you won't be touched.
Randy Tugman; the way to beat this guy is to use the layout. Once he starts to come for you jump over into either aisle. Then as he runs round to get you jump back over. Do this a few times and he'll eventually pause and try to get his breath back : this is the time to attack. Blade gloves are ideal and after a few hits he'll go for you again so just repeat until he's no more.
Slappy; you need to jump kick this guy to knock him over then you can get 2 or 3 hits in, again blade gloves are perfect for him. He'll start firing flames at you if you stay there so move away and repeat.