1) DO NOT sneak next to it unless your ambushing is at least Adept, maybe not then. If it spots you on the turn you move adjacent, it'll get at least two free attacks, and that may well be the end of the fight. Let it see you from a couple squares away - maybe make it chase you out of the lair so you have room to maneuver.
2) Don't ever charge it. If you do you will most likely bounce, fall, and die. End of fight.
3) Cripple its mobility first, as long as it can move at full speed it can potentially slam you and end the fight. Don't waste time attacking randomly. Keep backing away and wait for an opening with a very high likelihood of hitting & crippling injuring a leg or foot.
Remember, monsters often get free counter-attacks against missed called attacks, so don't take any shot with less then an 'easy' chance to hit until the creature's combat ability is impaired. If even those are missing then you need to take only 'easier' attacks, or use auto attacks, which tend to have a better overall chance of hitting, and less chance of triggering a counter-attack - but if you are reduced to that you are probably not skilled enough to be fighting whatever it is you have decided to take on. Now might be a good time to run.
Once the giant creature's mobility is impaired, you have the double advantage of it being unable to charge, and its attack skill is likely impaired by pain (though not as much as normal creatures, as a rule), now you can go about killing it. There are two basic methods of doing that.
1) Lung Shot. Take a long pointy stick and stab it into the creature's upper torso a few times until you mangle a lung. This causes bleeding, but more importantly it may 'wind' the creature and eventually cause it to fall unconscious. It's a particularly good way to deal with Ettins and Cyclopes. Once you land that hit, back away and have a sandwich or something. Wait for it to fall over then walk over and finish it. May not work on creatures w/o lungs.
2) Head Shot. This is the quick kill and is especially recommended for glass cannons like the Dragon, which is going to breath fire on you and kill you dead if you don't end it quickly. This involves dancing around your wounded prey, taking shots of opportunity to further weaken it while waiting for the head to come open. Use stabbing weapons for best effect, though crushing will usually work.
Hydras are nearly unkillable via most forms of damage other than individually killing all their damn heads. I'm not even sure the cursed things HAVE internal organs to stab, and if they do they are very hard to hit, even with a spear. They also love to grapple and tear people to pieces with those heads, so I really don't advise soloing them if you can avoid it - there's just no quick way to kill them that I'm aware of, though if anyone has found one I'm game to try it.
Another safety tip, when the description says that a night beast is 'average size' - that appears to be average size for a night beast, which seems to be a good bit bigger and tougher than a human. It's not an invitation to charge or grapple the damn thing. They tend to be very strong regardless.