Any level will do for this achievement, although the earliest levels will be the easiest. As for the kills there are two options. The first option is to equip a melee weapon either with a character that starts with one or by picking one up off the ground or buying them from the store. Weapons with swinging animations like the bat or fire axe can hit multiple enemies, however it is up to personal preference. The second option is to equip the Combat Knife card. This card will turn your melee attack that pushes back enemies into a knife attack that works with the same properties as a melee weapon. Kills with this card active count as melee weapon kills. Every normal enemy dies in one melee attack, so this is relatively simply on lower difficulties.