I think I would leave one rule. For instance you could take the highest enemy DR and add 1 for every additional enemy. So when you have a lion 9, a wolf 7 and a frog 2 you take 9+1+1. But when you roll 13 you kill a frog so there are only two enemies left and the DR is 10 now?
It seems quite interesting and may lead to some extremely difficult situations. Let's continue with your example. After killing the frog, then the player needs to roll a 17 in order to kill the wolf? If you follow this rule, it is just impossible to kill a lion unless you kill a frog and a wolf first.
Feel like you must scatter layer to layer in order to kill the core which is very cool and exactly what I want. This rule will require you to use the RC rule as much as you can in order to deal with the group of enemies or you may want to flee because the enemy is too powerful.
Sum up:
Group A
- Lion DS 9 (highest)
- Wolf DS 7
- Frog DS 2
The DS of this group will calculate like this:
Keep the highest DS, the lower will count as 1
Highest DS + 1 + 1
The DS of group A will be 9+1+1=11
- If roll 13 -> kill a frog (2), DS drop to 10
- If roll 17 -> kill a wolf (7), DS drop to 9(lion)
From now on, you can freely kill that lion.
I would't make it that hard.
Group A
- Lion DS 9 (highest)
- Wolf DS 7
- Frog DS 2
The DS of this group will calculate like this:
Keep the highest DS, the lower will count as 1
Highest DS + 1 + 1
The DS of group A will be 9+1+1=11
- If roll 13 -> kill a frog (2), lion is (9-2=7), wolf is (7-2=5), so new group DS is: Highest DS (7) + 1 = 8
- You roll 15 and kill'em all
it makes it harder to fight the group, because they get +1 for every additional member but it's still fast, because you only got one roll against all of the enemies.
In Advanced Dungeon Goons it works similar but you still got to fight against every enemy separatelly, so you they get +1 modifier to DS for every adittional group member, but you roll against every one. In our case:
Lion DS 9 + 2 = 11
Wolf DS 7 + 2 = 9
Frog DS 2 + 2 = 4
And you roll against a frog, you roll 6 and you kill it. Now you roll against a wolf that is 7 + 1 (because the frog is killed now), and so on.