If you find it hard to complete a level, do it the old fashioned way, upgrade your weapons/abilities further.
I won't know which weapon you favour, but what I did was to max Stamina, Self-Defense, Agility, which allowed me to survive most hordes, and to run away when things got too hot.
I maxed out my minigun and used just that. Bazooka's too slow, and even though it has good knockback/area damage, there are too many enemies to just kill with it. Flamer's range is too short. Sniper rifle can't deal with bosses.
My other piece of advice is to go for the zombie hordes - Easier to kill, and you won't have to deal with the irritating wolf mutants. You could also just gun down the hordes when they have a go at each other. Beats having to take down full health enemies.
Minigun is good, but I actually find the machete more useful for multiple kills. Armor needs to be upgraded, of course, as do the other things Nichodemus mentioned, but you can jump into a mob and deal death very rapidly. My record, without trying terribly hard, is 183. The one enemy that causes problems for me are the gunmen; take them out as soon as you can.
just "completed" all the quests, they are ALL SERIOUSLY BUGGED
received most of them randomly
and i sincerely hope there is no more decision game.. to me it was exactly like the others
Added to that, some just take up free slots and after adding that you might realize that adding that was a mistake and that it was not of great help. Further, after this you cannot cancel that Upgrade. This sometimes Frustrates me.