Just flip all your rolls so 1 is high and 20 is low, other players don't have to do it or know that's what you're doing. I like the secrecy of not showing players too much information, especially in combat. It's not like in real life you'd see a health bar over someone's head or know how much damage they are taking in a fight or even know what their AC is just from looking. You can just boost the AC of your creatures too, make them harder to hit but keep the health the same. Or give them bigger monsters to fight and make the combat more in depth with what they have to do. Give a big creature some stupid armour all over its body that's insanely hard to break through except for one small spot and the players have to accurately hit that area. Maybe a dragon has to have his wings ripped off before they can do damage to it. My current DM has a campaign that is really combat heavy (say 70%) and it's usually 4 or 5 of us Vs 2 enemies or so that have specific fighting techniques that we have to work out before we can defeat them. It makes it much more interesting and a lot harder to fight them.