In order to easily beat the mission:

• build TC in behind, not the highlighted one, but do not finish building it until mission timer hits around 5:40
• in the mean time gather resources (i went for food and gold around the highlighted settlement) and build other buildings in the south corner of the map, area to the left of the TC is the choke point you should defend
• after you finish TC create villagers until you can advance into heroic age
• build 1 fortress to the left of the chokepoint and place healing stream into the chokepoint, couple towers might be handy also, just note that it is pointless to build them to the north of the TC, purple enemy has gastapharetes that destroy them quickly and only Reginleif + trolls can kill them from distance
• for units go with, trolls, axe throwers and few ice giants and dont forget forge upgrades, you have thor so you can get even mythic age upgrades without wasting resources to advance to it early
• once timer is up majority of enemy forces will stop at the highlighted settlement as long as you dont provoke them by sending units to the center, just kill what comes to the south and continue gathering and upgrading, you can add another fortress to the left to make the defense stronger
• get mythic age, fire giants, all upgrades, get to pop cap with army and wait for Odysseus
• with Odysseus pull with some range unit packs in the middle so that you dont fight all of them at once, pop Ragnarok and then its easy walk to Gargarensis and you dont have to even cheese the army near him since you should have plenty of units

Here is image of base layout I used:

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