[WhYdOyOuThInKtHaThApPeNeD]

1 fairly important thing to consider when fighting the Bosses. Always use an Ancient Knowledge Wanderer Scroll before fighting the main Bosses (assuming you found one or more).

I like to use my own swarming strat against the swarming spiders. When I have enough Royal Thorns so the 3rd or 4th gen spawn makes a big enough stack to compete. Flood the field with Thorns.

Summoning spells and summoners like the Demonologist can help flood the field well. A mage with higher magic and powered up summon spells can bring in quite a flood, especially when combined with any of: Demonologists, Royal Griffins, Druids, Royal Thorns, Demons, Dryads, maybe Necromancers (but I haven't used them so I dunno how well they would work to summon flood). Not all of those will be as easily available unless the Spider is left till later.

I also like range units to strike the main Spider Boss, Cyclops are great, they beat down the summoned spider minions while striking the Spider Boss hard and they won't take any losses. The Demonologist has a pretty powerful range attack. If you have enough Druids their range attack can be good. Royal Thorns have a range attack. When flooding it's good to have range units and/or flying units or it may get hard to reach the Boss Spider.

Black & Red Dragons are nice to hit the Spider or Frog minions for their fire damage and ability to hit 2 targets. Then with the Red Dragons Fire Flood range attack for the Spider Boss, even though it has to reload.

I usually like to attack the Spider or Frog Boss when most of my units can kill the summoned minions with 1 blow. I usually leave them and come back to them after getting fairly well powered up elsewhere.
I think I had at least 50 Royal Griffins which is 15,000 leadership. I don't think I'd want to fight the Spider Boss with less than that. But I'm going from memory, you may want to have more than that.
I like to fight them as early as they are possible to beat without too much loss to get the most value out of the experience gain and the funnest challenge. Like DGDobrev said, hit the Spider, Frog & Driller boss in a row (when all other battles are cleared and access to each is totally open) after using the Ancient Knowledge scroll (so all 3 battles fall within the Ancient Knowledge Scroll effect), but when you're just powerful enough to beat them all. You should get a few levels out of just those fights. Have some other good fights in mind for the remaining 2 uses of the Ancient Knowledge.

The biggest experience scores I got from non-boss battles were the hero's:
Shiva: 6,652 xp
Fangol: 7,535 xp
Hagni: 5,472 xp
Demenion: 9,671 xp
Defense Diploma: 6,983 xp (with scroll)
Rakush: 5,097 xp

I would do the Spider, Frog & Driller before getting to the following:
Uchkuk: 9,259 xp (with scroll)
Shenobi: 13,505 xp (with scroll)
Ziraks: 15,016 (with scroll)
Harrum: 13,069
Gremlion: 29,921 (with scroll)
K'Tahu: 37,401 (with scroll)