One can also use tomes to permanently modify damage (with negative ones to decrease damage). Remember to remove the tome items after use as by default all items that die (used on pickup) remain forever and can be considered a leak.
One can modify damage within a fixed range by using item damage modifiers of various amounts up to a certain limit. For example if you make 32 different item damage abilities in twos compliment binary increments you can get a range of -2,147,483,648 to 2,147,483,647. 32 different abilities is not trivial however and the damage range provided is far bigger than reasonable maps will ever require so a smaller number of abilities should suffice.