30johner: you basically cant judge skill damage base on the raw damage data, as mob has resistance of their own, as well as affected by difficulty mode you are in. Take my own ranger damage as an example. Weapon damage shown in character sheet is bout 148-263 +(14-14), with 30+ dex at lvl31. The max normal damage on lvl28 black ring will never exceed 120 damage in high difficulty mode, and it can goes as low as around 70 to 90 damage per shot. If difficulty being tuned down, damage can be double up and even further more to the lowest difficulty.
Pure mage will be more suitable in maxing fireball damage, as they can just ignore passive like weapon mastery and concentrated all on spells related skills, since they can run around spamming magic missiles non stop. Explosive arrow will be a better choice, if you just need something to switch between whirlwind, where you get to spam skill more frequently. I not too sure if bow's damage and will increase explosive arrow damage or not, with highly chances it wont, as I tried with buffing up the way of ranger, the damage has no changes with explosive arrow. So I will assume it wont.
I actually did a small experiment by re-skill my lvl35 warrior with pure ranger skills and magic missile. And honestly it wasnt bad as I thought. All needed to be done is just stand there spamming hotkey for skills and potions, and you see everything around you went dead

Since my warrior only 35, its impossible to to get rank 13 explosive arrow as it need lvl37, and I has no gear which boost the skill. Rank 12 explosive arrow has 1804 damage as stated. It burst out 700+ damage on divine and Zandalor, where paladins take 800+ damage. Rank 13 magic missiles without any skill buff does bout 200 to 210 damage per hit on paladins and 170+ on Divine and Zandalor, with total 5 hits per cast. Its very funny to see both Divine and Zandalor keep healing themselves in order to survive my nuke of magic missiles, and finally both lay dead as their healing can match the speed of my spamming. Below are the screenshot of my re-skilled warrior and I dont actually had a good bow to test with him.