I'm having a little trouble with groups. Like, a single fighter, 3rd level maybe, with a good damage roll, can eradicate an entire goblin warband in one roll.
I don't know if that feels right.
You've probably alread thought of this, but maybe a group of monsters ought to have HPs equal to a multiple of the group size. Like: (vs a lone PC)
One (or two?) Goblin: Level 2, 5 HP, 1 Armor, 4 damage
A Crowd of Goblins: Level 2, 10 HP, 2 Armor, 6 damage
A Gang of Goblins: Level 2, 15 HP, 3 Armor, 8 damage
A Warband of Goblins: Level 2, 20 HP, 4 Armor, 10 damage
And then you do things like you do, but 5 damage drops it to a gang and then 5 more makes it a crowd and then 5 more leaves you with one or two.
Also, a "group" of one? That confuses me.