Level 7 (Seaweed Plains) is probably the toughest level of this game. I quit playing the game for a couple of months because this level frustrated me so much!!!
Once I got back to it, though, I finished the level and have since played it several times using a variety of strategies. Here's some general guidelines to take or leave and, perhaps, adapt to your own preferences.
You start with two bases but I never try to keep both of them going. There are just too many attackers to manage that. Instead, I upgrade the coral fort immediately on the left side camp, then (while the fort is upgrading), I switch to the right camp and move workers over to the left camp. I'm not a fan of mages, although they can come in handy in a pinch, so I'll move one mage, the gnomes and the upgraded merboys. I don't move all of them because leaving some behind tends to draw some of the enemies there and gives you some breathing room back at the left fort.
As a general overview of the level, there are really only 2 big enemy camps and one small one, but the big camps are big ones! The first biggie is in the northeast quadrant just to the west of the mounds, the second is just northwest of the right side camp in the cove created by the mounds. The little camp is in the far northeast corner of the map. There are numerous watchtowers around, as well, plus little pockets of enemies to surprise you (such as one in the dead end along the far east wall).
Back to general strategy, once your troops are grouped in the left camp, get the gnomes harvesting and move your tridents and mage to the western side of the boulder. This is likely where a big attack will come from (since the right camp will be a decoy for a short while protecting your eastern side). When the troops are moved, build a trident academy and upgrade it immediately. Train at least 3 scouts so they can build towers protecting your entrances.
With little time to spare, you'll be assaulted by a wave of enemies but your tridents and mage should be able to hold them off long enough to get a couple of towers built on the western entrance. At least 2 searing and 1 freezing tend to hold off the enemies enough, for now . . . Do the same at the eastern entrance. Build the towers out a bit, but not too far, to give yourself room inside the circle to build some hamlets as you'll need to do soon. With towers up, you'll have some breathing room to build hamlets and train tridents.