The easiest way is the level a bit further until the Evil City. Then unlock and max out the Gravekeeper Ghoul. The regular Ghoul from much ealier should be maxed out by now as well and both will give you a Stench Max of 60.
The base Stench you get is 10. 10 + 60 is already 70.
Next is to invest Sin on 'Inventory Creation: Cleaner', this will give another +10.
After that you can upgrade it another 30 levels through 'Inventory - Cleaner+', which gives another 30, although 20 are sufficient.
This gives a total of:
Allies: 40 + 20 = 60
Sin upgrades: 10 + 30 = 40
Having Smelly Satches will help filling the max Stench because it regenerates. Once you hit 100 the achievement will unlock.