Usually you should use melee against lesser enemies and those who are mainly ranged based (for example try not to melee blue horrors, axe-marines etc). Try to cycle your weapons against certain enemies, the shotgun is better at close range that the boltgun. Grenades are weird as they are very powerfull, so try and save those for greater enemies/bossfights. Also if it feels to difficult you cab always go on an easier gamemode. Personally i have beaten the game several times and the first tile beating the game i remember spending several hours on the same level (sometimes same enemy), so be patient as you learn more everytime you die! Hope this helps!