Well, other than questing and doing lots of FBs (and grinding/zhen/rebirth with a training eso) the only advice I can give is to have a good DD pet or tank pet who can take mobs without much healing. With the right pet, you can aggro several mobs at once and take them all. If you are a light or heavy armor build and aren't so squishy, you can kill multiple mobs with casting or fox form (the latter depends heavily on build) while your pet handles a few other mobs simultaneously. Most people tend not to try this because it is very easy to get killed if you mess up or aggro one too many mobs with adds like sacrificial assault. Keep your bramble guard up at all times and STAY CHARMED!! For this to work, you have to be charmed.
A veno with a herc can practically zhen by themselves if they do it right and with the right mobs. Picking the right mobs counts for a lot of it. You want something that gives enough exp without being able to completely destroy you or your pet.
I don't have a herc (and do not want one for various reasons) but I have found that a decent DD (in my case, a leveled varicose scorpion with tough, bash and pierce) can kill a good handful of mobs equal to his level reasonably fast with me casting in support. Also, something to consider is your pet's mag and phys resist. Scorps have high mag resist, so mag based mobs do very little damage to them. Magmites have high phys resist, so they can take more phys attack mobs. However the magmites do less physical damage and serve better as a shield while you cast.
Hope this helps. Btw, training esotericas are definitely your friend if you have a lot of mobs to kill in a quest.