As i have noticed no topics offering advice on this, so I will start one.

I have just heard that one of my opponents are planning of fielding a War Mammoth in their chaos army and am wondering what is the best bet to deal with this threat?

I know a couple of Items and combinations would work for example,

Virtue of Herosim and A great weapon.

Virtue of Audusicm (spelling - the one with if the unit has a higher strength I re roll failled hits and wounds) with the swords of Heroes.

Anything else which would help me here?

The Sword of the Lady's Champion or the Heartwood Lance would be good too, but as you say, the Virtue of Heroism, the Virtue of Audacity and the Sword of Heroes are meant to deal with such things. You have the right army: Bretonnians have a LOT of monster-killing things.

If you take the Virtue of Heroism, equip your Lord with Sirienne's Locket: the Mammoth won't be able to deal more than one wound at a time... and you'll be able, if you're lucky, to kill the beast in one turn.

If you take the Virtue of Audacity and the Sword of Heroes, use the remaining points for the Grail Shield to give your Lord a good defense against powerful attacks.