I was reading through the hotkeys and I am pretty new to Ladderslasher...

Is there a way to begin attacking a monster in the Catacombs via keyboard press or do I have to click the enemy?

You mean attacking the monster itself or to engage a new wave of monsters?
There is no hotkey to attack the monster itself, you have to click it. To engage a new wave of monster you can either whistle and you can set a hotkey for that or to engage and move to spawn new monsters is also a way. You can set hotkey to engage

That is what I meant, but I appreciate your additional help and tips. Thank you!

I think I found a good way to play now

There is one thing that was missed out in that explanation... Press "ESC" and you will come up with 3 tabs, and the first one is "General Settings"... There is a tick box "Disable Auto Hold Attack" ... what this means is that normally you can hold down the mouse button and it will auto attack (as long as you hold button down)... now if you UNTICK this, you can click ONCE on the monster and it will Auto attack, now what is not stated, is that you can press the Charm(main) or Acc(charm) slot to either cast or heal while attacking... basically it means "Auto Action" = "Healing, casting or melee attacking" if you choose the Base hotkeys Q (weapon), E (main charm slot) and R (ACC slot, normally for healing charms).

Oh, and while you are auto attacking/healing etc, you can move your cursor over dropped items and read them and click on what you want etc.