Walkthroughs for Campaigns and Maps in WoGA.

only one high archer remains; on the second island with one archer, I can't kill the gremlins until the path to the troll opens up

That's your payment for leaving your friends on a sinking ship. There are hints there at every step.

I have a question regarding the "All or Nothing" campaign. All searches for its description only lead to questions from other players. Therefore, I will share my own achievements. So far, only for the 1st map.
1. The objective is not entirely clear. The briefing says you need to reach the capital, but nothing specific is mentioned, while the saves say "collect resources." In the end, using the artifacts I found, I managed to reach the upper right corner of the map, and the objective was considered completed with the words "you have collected more than 30000 wood." It is logical to assume that the victory condition is collecting wood or reaching the upper right corner.
2. To collect 30000 wood, considering that its only source is the resource tower in the Castle, you have to spend just as many days after capturing the castle. Not everyone will have the patience to play that long. Perhaps after completing the forester's quest (kill all the undead in the forest), you can receive a lot of wood at once or some other resource source. But so far, I cannot complete this quest because the number of undead grows much faster than my army.
3. The option of reaching the upper right corner of the map is feasible, as in my case using artifacts (I came across "Angel Wings" and "Water Magic Tome," and I could cast "walk on water" even without a spellbook), or after completing some other quests. Which ones exactly is unclear.

Further on the map Walkthrough.
1. The hero level on the first map is capped at 15, while there are so many different monsters there that the ceiling is reached very quickly. Therefore, you should always take gold from chests and buy an army.
2. First, at the crossroads, turn into the village, talk to the elder, and enter the city via the market.
3. In the city, you can hire troops. The number of troops is unlimited, but they are expensive and the purchasing process is very cumbersome. This suggests that later on, the main source of troops should be different.
Talk to the mayor, find out about the criminal, walk around the prison, respond to the call, and receive a quest to help the prisoner. Go back to the mayor, tell him your plan, and receive documents from the mayor. Go to the prison and take the prisoner. He leads us to the thieves' den, where we get a quest from the thieves to kill the mayor. Return to the city, collecting resources and wiping out monsters along the way. Next, you can capture the stone mine; it is located slightly above and to the left of the northern city entrance. To the southwest of the mine is the cliff of the green dragons. You should take it as early as possible, as it will be the only regular source of decent troops. In the city, you can either kill the mayor or tell him where the thieves' den is. This might be a script flaw, but I didn't notice a difference in subsequent events, except that in the second case, a quest to wipe out the thieves appears. The mayor will send you to the sheriff. When you are ready, the sheriff will teleport you to the thieves' den; there will be 500 of them, and after victory, you are teleported back to the sheriff. Amusingly, after defeating the thieves, if you visit the mayor again, the hero first kills him, and then the mayor thanks you and says that you are an honorary member of the city...:)
4. Near the western city entrance, there is an entrance to the swamps; go through the swamp to the cave, kill 4 ogre mages there, and rescue the forester's daughter. Take the daughter to the forester; his house is located exactly in the middle of the cursed forest (filled with undead and located in the western part) and is guarded by one black knight. The forester will give 5000 gold for the daughter. Ask him to resume wood supplies to the village; he replies that he cannot as long as there are undead in the forest. I don't know how to wipe out all the undead.
5. Slightly west of the swamp is a cave entrance, guarded by 5 red dragons at the top and 3 black dragons at the bottom. Wipe out the dragons and 1 dark dragon (level 8). If you have enough strength, you can get another dark dragon, but there won't be much use for it; the only thing is that it inspires fear.
6. Go to the village to the elder; he thanks you for ridding them of the dragon. Then go to the lighthouse; the keeper refuses to talk and sends you back to the elder. Go along the road between the lighthouse and the market, and barbarians will appear. Wipe out the barbarians (1 behemoth, 1 thunderbird, 2 ogre mages, 10 goblin wolf riders, 15 hobgoblins) and get in their boat. In the boat, I recommend sailing around all the water and collecting artifacts.
7. Go to the upper left corner and capture the Castle (5 ancient behemoths, 20 ogre mages, 60 goblin wolf riders, 100 hobgoblins). There is a capitol and a grail in the castle, so there will be money now, but still no troops.
8. Return to the lighthouse; the keeper turns out to be a carpenter, who thanks you for ridding them of the barbarians and hints that it would be good to find a toolbox (read "grail," and the grail from the city doesn't suit him, you need to find another one).
9. I'll say right away that the grail is on the island with the university and emerald dragons, right under those very dragons; there are 5 of them. To find this out, you need to remove 3 obelisks located in 3 different dungeons. The dungeon entrances are on three islands and are guarded by super phoenixes (5 each). Besides them, there are many monsters and war machines in the dungeons. I managed to remove all the obelisks only because I found "Angel Wings" in the very first dungeon and was able to fly. I don't use cheat codes, and I don't know how to pass it honestly—the monsters constantly multiply.
10. Give the grail to the carpenter at the lighthouse; in exchange, he gives you a pickaxe (not visible in the inventory). Go to the city, to the lower right corner; using the pickaxe, a passage to the mine opens, through which you enter a dungeon. After passing the dungeon (again, crowds of monsters), pick up the spellbook and learn 8 spells of level 1-2.

These are the results so far. If anyone has found anything else, please share.