Have been using TP Foe to successfully >50% to pull singles from groups. Always target the Lt or Boss first.
The senior ranks will always protect their minions, but does not work the same way. E.g. the zombies do go 'lifeless' if take out the Boss, Lt, Reapers, Morts.
Main aim is to pick off the edges of the groups, behind pillars, etc. IMHO line of sight has a greater impact - hence TP foe being used effectively from Max range AND round corners.
4 heros vs 1 Boss or Lt = a very quickly arrested mob. Even if the others are on their way - you have time to heal/recover somewhat.