When asking for help on quests, you really need to let us know which upgrades you have, because the strategy may be different for someone with chicken punishers or with towers, for example.
The first part of fried oysters, you need to prevent the enemy players from getting wood from the forest in the centre... no wood = no army. After you've built your quarters at your starting point, you can choose whether to just go with Tygress' peasant rush idea, remember to station a handful of peasants between the forest and the base you don't attack first.
OR you can send a few peasants to guard each side of the forest while you build an armory and raise an army of whatever warriors you happen to have available to you. Once you have a few warriors, replace your peasants on guard duty & get them working on gathering resources so you can produce enough warriors to easily overcome first one enemy base, then the other. This strategy will work no matter which units you have at your disposal, it's just A LOT faster if you have berserkers and/or chicken punishers.
I suggest that even if you can win a level with a peasant rush, that you instead produce an army of axemen and/or berserkers etc just to get used to gathering resources and producing warriors as these are skills you will definitely need in later quests and on multiplayer islands.
The second and third parts, i agree with Tygress, send about 10 or so warriors to your partners bases and then concentrate your attacks on the nearest enemy to you, eliminating them one at a time. Keep an eye on your allies bases and divert troops there if necessary, but they can largely take care of themselves. Once you have defeated one enemy base, the odds are more in your favour and you should have no trouble finishing off the remaining enemies