I got to level 14 in Nethack, the furthest that I’ve gone yet. It sure gets darn tough past level 10 or so. If you are lucky enough to find a scroll of genocide, it is good to use it against tough monsters. Well there are sure enough monsters that fit the “tough monsters” category so I can’t say for certain which monsters you should genocide but I can say that the first monster types that you encounter that you may find some serious difficulties with are ants, whether they be fire ants or ants of another type. Also Killer Bees are tough too. Mainly because they usually come in groups similar to ants and will swarm and attack you and they are pretty tough to kill especially when fighting one after another after another. Usually when you first see an ant or a bee, just make a bee line to the nearest exit of escape. Pun intended. There will definitely be more stronger monsters that you will want to genocide like Titans on Medusa level.
There is also the difficulty of what the heck to do with the Big Room. Jeez that’s tough. It’s not a definite that you will encounter the big room but if you do it would occur on levels 10-12. Hopefully you don’t encounter it cause its a real bitch.
I got killed by a Mumak on level 14. Don’t let these Mumaks get too close cause they will clobber the hell out of you if so. Hit them with ranged weapons and wands/spells.
You can also change the tileset display to various graphical versions. I like the default version the best of the graphical tilesets but I am most familiar what monsters and items are what with the non-graphical ascii version so I prefer to use ascii over the graphicals, but the default does look very nice. Below is the default graphical display of Mead and Sirius the dog meeting the Oracle in Delphi.
On a non related topic, David Bowie’s Sense of Doubt from his Heroes album is amazing!