When shooting at a large monster and its rider roll to hit the model as normal, adding +1 because the monster is a large target. Once you have established how many hits have been scored you must apportion them between the rider and the monster. For each hit scored roll a D6:

Work out wounds separately on the rider and his mount. Take any saving throws due to the target as normal. Most monsters do not have an armour saving throw as they have no armour, but some have scaly hide which confers a separate save. Riders are permitted saves for their armour, but remember they do not receive the additional +1 as do cavalry riding smaller creatures.

If a monster has two or more riders (a very unusual combination) then randomise hits to see which rider is hit and work out any resultant wounds on that individual. If models are glued in place (as is likely) it will be necessary to make a note of any casualties suffered.