In my current game I test the droid army. I have, or will have (used save game scanner) the following items:
Axe of Ice, Class III
Royal Hammer
Mithril Shield
2 x Slippery Curiass
Twinkling Boots
Engineer's Badge
Horn of King Borg
This will give 10+15+20 = 45% Leadership reduction and 10+15+2x10=45% Physical Resistance from items. Add to that the 7% from medal and the 20% of the droids, it adds up to 72% physical resistance before Stoneskin is used. So in this game not only the guard droid, but also the repair droids are used as tanks
The only companion that allows these items to be equipped at the same time is Jimmy Kraud. (At least when you play as a Paladin as I do)