Something similar to that which requires the forester but doesnt require the forester.
Anyone who enters the field gets 3 creepers and 2 skeletons every 10 seconds spawn around them

/ Mobs-on-entry
/ - Creeper 3
/ - Skeleton 2
/Interval 10

what do ya think?

Sorry for the / , it wouldnt show it correctly otherwise

Although this may not be the perfect solution for your needs, you could do this fairly easily with some Redstone and command blocks. Just make a clock with a 10 second delay hooked up to a command block for each monster you want to spawn every 10 seconds.

Command block 1
/execute @A[r=15] ~ ~ ~ /summon Creeper ~ ~ ~ {}

Command block 2
/execute @A[r=15] ~ ~ ~ /summon Creeper ~ ~ ~ {}

Command block 3
/execute @A[r=15] ~ ~ ~ /summon Creeper ~ ~ ~ {}

Command block 4
/execute @A[r=15] ~ ~ ~ /summon Skeleton ~ ~ ~ {}

Command block 5
/execute @A[r=15] ~ ~ ~ /summon Skeleton ~ ~ ~ {}

This would do what you asked above, but the only down side is the monsters will be spawned directly /on/ the players within the area You can spawn them at an offset but it'll be the same offset every time unless you want to make this a lot more complicated. As /execute will fail when no players are within 15 blocks of the command block executing it, no monsters will be spawned unless someone is within 15 blocks of it.