There are several excellent arthas models on this site. Download one of them.
Open up the world editor (located in your Warcraft 3 folder) and go to file->open. Find one of blizzards melee maps and open it. Then import the arthas model you download via the import manager (F12). After that, open up the object editor (found in the module tab of the main editor screen) and go the units tab. Then pick a unit (say the paladin unit under human->heroes) and find where it has it's model path. Click on the model path and find your arthas model in the list of imported models.
After that your done. Save, exit, and open up the modified melee map in warcraft 3.