Old topic, but I've refined my system a bit (I described how I was doing it back on page 2), and I'm really liking the results.
The assumption is difficult situations: ie, theater of the mind, with lots of mooks, maybe even some of different types.
Using some standard lined paper, I make 4 columns. Sometimes I do it ahead of time, but it only takes a minute, so it can be done on the spot. The first column lists the number of monsters. The second column lists the designation for the monsters. The third column lists which PCs are engaged with those monsters, and the last column (the long one) is for tracking damage.
First, divide up the monsters into manageable groups. If you are using mob rules from the DMG (which I highly recommend--its a real time saver and still keeps a satisfying feeling to the battle), you might take that into count when deciding how many groups to split them into. The designation for each group needs be clear and easy for you to visualize the general position of. For example, if there are 40 goblins, and a goblin boss, I might have:
10 | NG |
10 | SG |
10 | EG |
10 | WG|
1 | GB |
It's not much trouble to keep in your head in general terms which group of goblins the PCs are attacking. So let's say we have 4 PCs: Anna, Bert, Caltor, and Dirk. I indicate who is engaged with each group. Note: it is possible to be engaged with more than one! So that's what our third column tells us.
10 | NG | A
10 | SG | B
10 | EG | A
10 | WG| C D
1 | GB |
If characters move around (or groups of monsters move around) just cross off and write in your A, B, C, or D as needed to keep track of who is currently engaged. I've found that that column doesn't need to be that wide.
Then, in the final column we are keeping track of how many hit points have been dealt to individual monsters. So let's say a weak area effect goes off on 5 goblins in group NG. Three of them save and take 2 damage, while the other three fail and take 4. It would look like this:
10 | NG | A | -2 -2 -2 -4 -4
Since there are 10 goblins and only 5 numbers, we can see that 5 goblins remain undamaged.
When one of those monsters is dealt additional damage, we cross off the current value add the additional damage, and write in the new value later on the line.
10 | NG | A D |
Still 10 goblins alive with 5 wounded, and Dirk has joined the fray. Slashes are completely ignored once they are slashed. The only thing that matters are "-#" and "X". X is what we are going to do when a monster is killed. Dirk goes ballistic on his turn, Action Surging or what have you.
10 | NG | A D | X \ -2 \ X X -5
We now have 3 dead goblins and two other wounded ones (the current -5 was the second -2 that Dirk wounded after taking out the assorted three goblins).
Now it's the goblins turn in initiative. The first thing we do is a quick clean up. We subtract the dead goblins from the number of goblins, cross it off and write in the new number. We draw a horizontal line through all of adjacent Xs and \s so that the only things we are looking at in the damage section are the currently damaged goblins.
7 \ | NG | A D | ----- -2 ----- -5
Now the 7 goblins take their turn (using mob rules if possible), rinse and repeat.
Once all of the goblins are dead, you just X off the NG.
Another thing I've started doing is to write the -# values on the top part of the writing space, so that I can make notes underneath, such as a P for prone targets, etc.
The DM is free to decide which of the targets a PC hits, either having them focus fire or randomly distributing attacks, whatever works best for your group.
It's working quite well for me, though I'm always trying to find little ways of further refinement.