Easiest to play : buff and debuff mage. (with the exception of the first 4 islands, where direct damage is better and you don't have too many spells anyway)
Your level before boss spider: 19-20 (impossible diff)
Your level at end of game : 65-70
For the boss spider, it is very important to have at least 1 point put into Frenzy skill, from the Might Tree. It increases attack every time your units kill units. The spider will have lots of summons and your units' attack will greatly increase by killing them. IF you can get Transmute, it will help with regenerating mana during the battle. But it is hard for a Mage to find so many red runes...
Against the spider boss I'd go for direct damage. I could deal 1400 to the spider boss/round with 2 Magic Missiles. I doubt summons could manage this. Call of Nature if I'm correct is also expensive spell at lvl 3, and mana will run dry fast with it.

I am playing on hard mode. I reached the spider on 18 level. Try to open all chest on battlefield. You may success to find useful scrolls. I killed it by use of fire arrow (it has not resist to fire) and 3 troll. Afterwards the easiest way to play to cast phoenix call up to 50-60 level. But you'll have all magic schools to such levels. Good luck.

What about leadership? Will I get in trouble with the spider boss if I don't take Glory or Jarl? Should I take Glory or Jarl before I fight the spider boss?

Try carefully to spend your runes. Personally I spent them actively only on first island because it was hard to play without losses and magic helped much. But then I tried to accumulate runes to battle with spider because much depend on what magic (scrolls) you'll find. Much depend on current situation. Personally I used "glory". Frankly speaking it will be enough leadership to buy 3 trolls and kill the spider without losses. Decision on you.

If you care about playing without losing a fight, and without reloading in case that you lose, (aka playing hardcore), you can take both of them. The extra leadership will help kill the spider faster and will mean a lower risk of losing the battle.

The spider can, however, be killed without having any of these skills, even with the mage, even on impossible. So you will not be in trouble if you don't take them. It might just be a little safer with them.

Well, I do always try to play without losing any troops. Thanks for answering, guys. I think I won't be taking Glory for now. Maybe later when I need it. I'll try to get Higher Magic first after I get my Rune Magic to level 3 and try to get Frenzy and Learning when I can.

Keep in mind that the spider has some resist to Magic Missile.

Roger that. Hopefully, I'll find more scrolls in the next island.

It may be better to wait until just before the Spider boss fight to decide which path you want to take. If i were you, i would only invest runes in skills that are absolutely needed to continue.
You may decide to go down the Rune magic path and in one of your last battles before you meet the Spider, dig up a Summon Phoenix scroll......:grin:

That's a quite interesting idea, travelingoz. What skills did you invest in when you're in the 1st island?