Familiars = overwhelming recon advantage
As others point out, coral reefs take a long, long time to build. Even with magical help, I don't think the Jongans have to worry about coral.
The coastal defense trees are another matter. So, the Jongans learn to NOT row directly to Cresia, but instead to row along the coast until they find a clear landing area.
Coastlines are long, and extremely difficult to defend in force. Especially considering the limited population and lack of long-range communication. Remember, all communication at this age must go by foot--it would take several *days* for news of an attack on the coast or borders to reach the interior. And by that time, the attackers would be long gone.
It would be a simple matter to row a couple miles along the coast, land, and completely avoid the Cresian coastal defenses--whether those defenses are trees, coral, kelp, or massed sea creatures.
Once on land, the Jongan forces begin to use their Secret Weapons: flying familiars. Aerial reconnaissance is an extreme advantage in warfare. Especially when your aerial reconaissance can see in the dark (+14 Spot for Owl). Familiars can also be used for communication between groups. Ravens can speak, after all.
It's unlikely that the Cresians would be wary of flying birds, and even if the birds were attacked by summoned flying foes, the tougher Familiar could just retreat back to the safety of the Jongan force (by a circuitous route, of course).
So, using flying familiars as scouts, several stealthy Jongans raiding parties land and are able to avoid masses of Cresians. The familiars are able to locate the Cresian Druids and Clerics as they go about their daily lives--or rest at night. The Jongans act on this information and assassinate the Cresian clergy with lightning night raids. They pick off any survivors who might try to run and give warning of the attacks (familiars delivering Shocking Grasp touch attacks, or Magic Missile from afar). Each small raiding party should be able to slay many clergy before they're forced to retreat (again, using flying familiars to spot and thereafter avoid approaching Cresian reprisal parties). The Jongans burn as they go, using the fires as cover and as a distraction (forcing the Cresians to chose between pursuit and fighting the fire/rescuing people).
If the Jongans are clever and make sure to have their familiars act "natural" (as in, the Jongans never have the Familiars hang out with their Jongan masters), the Cresians won't even have a chance to but two and two together--they'll never make the connection between Familiar and wizard.
A few such raids will quickly deplete the Cresian casters, forcing the Cresians to group their remaining casters in a few fortified locations. Without widely-dispersed magical support, the Cresian defenses are severely compromised. The Jongans can resume viking raids and even larger-scale attacks, using their familiars to warn when the Cresian Druids are finally alerted to an attack and set out from their fortresses.
Once the druids come out of hiding to respond to the raids, the Jongans spread out and surround the druid force, using Magic Missile and tried-and-true Summoning spells to pick off casters from range. They also use Faerie Fire spells on enemy casters, giving the archers specific targets to shoot at.
It won't be long until the Jongans have an overwhelming magical advantage as well as their already-established military advantage. Eventually, the Cresians will retreat into a few large, well-fortified cities. The Jongans establish colonies on the coast, gradually take over the land, and either force the Crecians out or absorb them into their culture... although ironically, the stronger Crecian religion/peacetime way of life may overshadow the shallow militaristic culture of the conquerers.
-z
PS: the druids' Animal Companions and Speak With Animals spells are not anywhere near as good as Familiars, since:
1) animals are dumb: Int 2.
2) animals don't understand human language.
3) animals don't have the empathic link or Speak With Master.
4) animals can't follow complex plans
5) animals can't adjust plans "on the fly"
6) animals can't report/receive new orders during a mission
7) animals are unreliable, have trouble with concepts such as time, can't really differentiate between unfamiliar individuals, and so on.
Example:
Druid (casts Speak with Animals): Hawk, my Animal Friend, fly and look for enemy troops, then come back to me!
Hawk: (Flies away. Comes back in 1 minute. Or 2 hours. Or 2 days.)
Druid (casts another Speak with Animals): What did you see?
Hawk: Men.
Druid: Jongan men?
Hawk: Men. Men are Men. Saw men.
Druid: Did they have weapons?
Hawk: Weapons?
Druid: (points to sword, spear of companions) Weapons--like these?
Hawk: Yes.
Druid: Where were the men?
Hawk: Not here. Away.
Druid: Yes, but where?
Hawk: By tree. In field. (indicates general direction by pointing beak).
Druid: (looks around at hundreds of trees in nearby forest) Argh! Okay, lead us to the Men you saw--we'll follow you.
Hawk: (Flies away, leads Druid to a group of Cresian farmers wielding shovels and hoes)
Hawk: Scree!
Druid: What?
Hawk: Scree!
Druid: Oh, my spell wore off. (casts another Speak with Animals). What?
Hawk: Men here! Feed me now?
Druid: Argh! Well, it's getting dark. Let's camp. Hawk, you stand watch.
Hawk: Watch?
Druid: Start calling if you see any Men approach.
Hawk: Okay. (guard walks up to help Druid set up camp) Scree!
Druid: No, no--yell if a Jongan approaches.
Hawk: What is Jongan? Mouse? Mouse tasty.
Druid: (sighs) Just guard me--like the Trick I taught you.
Hawk: Okay.
(Later, a Jongan owl familiar spots the camp. The Jongans approach and attack at night--some mages use Dancing Lights to illuminate the camp for Jongan archers, while others use Burning Sphere to sow panic and burn tents. The Crecians have no targets for return fire. The Druid comes out of his tent, the owl Spots him and his master hits him with a Faerie Fire. The archers and other mages concentrate fire on the Druid. Once he's down, they mop up the others.)
Hawk: Scree! (flaps off into the dark, scratches an archer, archer grabs and crushes bird)
Jongan archer: Check it out--new feathers for my arrows!
Raven familiar: Nice! Stupid bird.
Owl familiar (laughing): Hoo! Hoo! Hoo!
Raven familiar: So where to next, my friends?