In Shatter, only magical damage is halved on the icebound tomeliths. Apparently it was due to casters being able to be really safe while DPSing the tomeliths, but we all know it's because the JP meta before nerfs included parties with lots of SMNs and BLMs that can nuke down the ice really fast at full damage. Physical ranged DPS (BRD and MCH) are unaffected by it and do full damage to ice. Other sources of magic damage are also affected, such as NIN's Raiton and lots of DRK skills that I can't recall but do magic damage.