There are a total of 7 entrances to the base, place a fireball tower on every each one of them.
After that, put a blockade in front of all 7 fireball towers. That should be enough defense for the first wave.
Some of the choke points will need 2 blockades, fill them up while you're gathering mana from the mission or after.
After you have enough mana, put up a DST over here, facing the crystal.
Put a second one, facing the other crystal.
And put a third one for the third crystal.
During the next wave, you should collect enough mana for a total of 7 lightning towers. Build one at every entrance.
I also put up an MMT over here.
And one on the opposite side. It would help with the ogres.
When you have the mana, go to one of the 3 ledges that have catapults around them, and put a DST facing this direction, it will help in the future.
Another one over here. You can see the places i'm talking about on the map, to the left, to the top and to the right.
Third and final one over here.
They might destroy your DSTs, just remake/repair them.
The next 2 waves can be REALLY tough with the incoming yellow ogres. Try to stay there and upgrade/repair your defense as much as possible.
Also, make sure you keep track of your towers and repair them regularly, they will take a bit of damage here and there and it will be really important not to lose them.
The last wave has a few ogres, which makes it a huge pain in the ass.
Try to repair everything the ogres lays a touch on.
After the last wave, it gets problematic as you can see. You will have 15 seconds, as per usual.
This boss is a huge pain, you have to use the balistas against him.
After you have done so, he will land on one of the spots i discussed earlier, that's what the DSTs we built earlier are for. He will destroy them eventually, so don't expect an easy kill, repair them and rebuild them when he does destroy them.
Watch out for your base though, make sure your defenses do not get destroyed while you're off chasing the boss.
My last battle with him was pretty intense.
My defenses started getting destroyed, i started losing HP to his attacks and i thought i would fail the mission.
I barely made it through, but i did. You should get 2 items, and an achievement.
You will get to the ending screen, but there is no next level button. You finished the main campaign, congratulations. There is much more in the lands of eternia though, unlocked as DLC, or you could try some challenges that do not need any DLC to be unlocked. In any case, good luck!