How about these as a rough guide for lumping:
1) Take strongest (highest CS) character
2) other characters OF SAME TYPE contribute 1/5 CS and 1/3 EP (all fractions rounded down)
Eg. 3 Giaks of CS 14/13/13 and EP 17/18/17
Base: CS 14, EP 17
+ second: CS 13/5=2, EP 18/3=6
+ third: CS 13/4=2, EP 17/3=5
Final target
3 Giaks: CS 18, EP 28
FIGHT SEPARATELY:
If you fight them separately, each Giak gets +4CS. You will take more damage per round than against the lumped opponent, initially, but as you kill off the weaker opponents the other opponents will see a corresponding drop in their CS - which will then be reflected in a lower damage taken per round as well.
FIGHT LUMP:
You will take less damage per round (one CS 18 opponent instead of three opponents at CS 18/17/17), but will still take high damage right until the last round as there will be no drop in CS as total EP drops.
Another example could be from LW 10, the first Death Knight you encounter under the bridge:
Death Knight CS 24, EP 38 (section 180)
Lumping contribution:
- CS = 24/5=4
- EP = 38/3=12
So 3 Death Knights: CS 32, EP 62
Compare this to LW 10-320, when you engage the following:
Elite Death Knights: CS 42, EP 48
This gives no indication to their number, but the following note can be made: if you chose to shoot one with your bow, their stats will compare thus:
Elite Death Knights (no bow): CS 42, EP 48
Elite Death Knights (1 shot): CS 38, EP 45
This implies an Elite Death Knight contributes CS 6, EP 3 to his mates. EXtrapolating backwards to the Death Knight under the bridge:
- CS bonus = 6/24 = 1/4
- EP bonus = 3/38 = roughly 1/10, rounded down.
Revisiting the Giaks and the Death Knights with these ratios:
1 Giak: CS 14, EP 17
3 Giaks: CS 20, EP 19
1 Death Knight: CS 24, EP 38
3 Death Knights: CS 36, EP 44
4 Death Knights: CS 42, EP 47
In light of these numbers, I would then suggest the following 'rule of thumb' guides:
TIER I targets: (like Giak)
- 1/5 CS, 1/5 EP
TIER II targets: (like Death Knight)
- 1/4 CS, 1/10 EP
So the higher 'level' the target, the more CS and the less EP it contributes.
