http://img.photobucket.com/albums/v260/N...le0021.jpg
There's a picture of a couple dragons in action, at Zakum. All dragons I have seen do between 1k and 3k damage, which is not that much, and they do not attack frequently.
Priests max Holy Symbol first, to increase their training speed. After that, typically elemental resistance. Priests basically just follow Dragon Knights (3rd job spearmen) and use HS and heal on them over... and over... and over when training at Grim Phantom Watches. The priests that train at Vikings climb up and down repeatedly casting HS and Bless. They don't actually do any killing, and because of that, they rely entirely on the rest of the party's kills for their exp, meaning that they gain their exp more slowly.
However, Viking parties are formed around the priest, and a priest can very quickly and easily get a full party, while it's a pain for me and other non-priests to find a party quickly. Priests can also go to Zakum earlier than other classes, as they are there to heal rather than attack. I've heard that priests training at Grim Phantom Watches gain exp much faster than those at Vikings, though it is more dangerous, and if you lag, you'll die.
Summoning Stones cost 5k each, and you could probably afford to use it, but it wouldn't be worth the cost unless it was maxed. To max it, you'd have to sacrifice the opportunity max a different 3rd job skill, such as Shining Ray or Doom.
Hermits, Rangers, and Snipers all use summoning stones in 3rd job as well. Rangers and Snipers use them to summon birds that act the same as dragons, and Hermits use it to summon a Shadow Double. Shadow Double follows behind them and does half of the damage that the Hermit does again by throwing two extra stars per Lucky 7's attack.
EDIT: Here are a couple more screenshots that may help you in your decision. I was about the same level as the fire mage using explosion in this picture (I am Ice/Lightning). Her explosion does about twice the damage of my Ice Strike, however, it is much slower and does not freeze.
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