How can I maximize damage reduction?

Klorox

I’m looking at using either a barbarian or dwarven defender as a point man to attract physical attacks.

Grond0

The dwarven defender special ability provides 50% and its natural resistance at high level 20%. If you use the Defender of Easthaven flail that's another 20% for a total of 90%. There's a helm that gives you 50% against crushing damage, so it's easy to push that over 100%. You can also get over 100% by using the rat form from the Cloak of the Sewers.

A barbarian also has 20% naturally at high level and can add 40% from hardiness and 20% from the flail for a basic total of 80%.

Classes that can use priest spells (like paladins for instance) can use Armor of Faith to get 25% resistance - add hardiness and flail for 85%. However, that spell can be dispelled.