A lot of people don't deal well with percentages, to be honest. A quick and dirty method for mass use might be something like '-2 defenses, +2 attack', but as noted that also makes status effects far more common.
is it feasible to cut hp by half and double the number of monsters?
If you are going to do that, then I would recommend using more minions instead. They have their damaged toned down so as not to overwhelm the party with additional flank attacks and damage. Otherwise you are putting too many opponents in the mix and everyone is going to have a couple of special friends flanking them and pounding them into the ground.is it feasible to cut hp by half and double the number of monsters?
You are correct, math FAIL. Removed previous post.How are the two different?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.