Swarming has been nerfed to the point where its not really that great.
It is better to have few High level toons and just kill in lower level zones.
For example
If you are leveling a 90 toon , have him grouped with 120's and go some where you can kill fast , Like Frontier mountains
this is the best way to do it, I know not everyone has this luxury of having toons with 30 level gaps but its worth doing.
example : I have level 90's toons to help level 60 toon , I have level 105 toons to help level 75 toons , I have 100 toons to help level 70 toons
you get the idea