The wall's primary benefit is battlefield control.

Example: The enemy army has a large stack of a powerful ground unit, say knights. Place a wall so that the knights can move around it, but doing so eats up one or two turns of movement. The knights will almost always attempt to go the long way around, rather than simply smashing through the wall.

Also, late in a battle, when the enemy has a very weak stack, you can trap it behind the wall while your inquisitors refill your rage, and your other troops run around grabbing mana and rage from the chargers you summon. Tedious, but effective.

For myself, I've also found the sword and underground blades abilities are useful spellcaster killers. Used against priests, inquisitors, mages, etc., they generate a good amount of XP (so you can upgrade your Zerock abilities). Underground blades also has the advantage of being usable even if your own troops are nearby, since it only affects enemies.

I prefer to boost underground blades as much as possible, then sword and wall in roughly equal measure. I completely skip rockfall.

All that said, Vulture's strategy also works well. One of the best features of this game is that there is no ultimate strategy that trumps all others.

Rockfall doesn't harm your troops, too.

@Sir Whiskers: 100% agree with your explanations of the Rock Wall. About the Sword, Blades and Rock Fall I can only say: until level 15 - tops. After that it's wasted rage. When I said that they're useless I was viewing that from an endgame PoV.

My playstyle for instance relies heavily on Time Back, Chargers, Ice Ball and Ice Prison (for Dragon Fights). Casting any other rage ability is out of the question most of the time.

@msp: nor does it harm the enemy (much) xD

P.S: Early on I like boosting Evil Shoal as far as possible. It's my main damage dealing rage ability throughout the Islands.

I like Evil Shoal too, but when I have enough rage for it, the enemy is already split through the battlefield and it's hard to hit many enemies.

The Ice Ball is nice, but it's har to hit enemies with it, I like Gizmo more...

Where is that clock I must find for Reaper? In Taron Mines?

The Ice Ball is not for damage purposes, it's for tanking, distracting and blocking archers.
Questions concerning the quests have been answered numerous times (more thatn I can or want to count) ad nauseam in this forum. Use the search function, please. There is also a very detailed walkthrough on the internet (linked somewhere on the forums) that gives you all the information you need.

Just a personal hint: as soon as you get the quest for liberating Castle Bogacho for the clock, do it. Every opponent killed without The Reaper gaining experience greatly decreases your end game performance. Especially - but not limited to - on higher difficulties.

I would do it if I there weren't so much higher level heroes around it... I'm only at 16, and there's a lvl. 24 hero unit...

Forget about it, it's done

I ask this in order to avoid the "use search" statements. So:

Would it be fine to ask here about spirits' spells to max out, so they can fit my play-style?

What exactly is your question ?

You can max the rage spells you like to use. And that's even as easy as it gets and as intuitive as it gets. Just use those you prefer the most and it will be very likely that you'll be asked to upgrade them.
A general rule of thumb is: restorative spells like Time Back generate experience for the Spirit according to the level of the Troop it's cast on, the number of units restored with it, the level of the spell and/or number of charges of it removed or restored. Damaging spells of course generate experience for the spirit according to the number of kills and the level of the killed units.
Support Spells like Chargers gain experience accoring to the overall leadership cost of all enemy units still on the battlefield. That means if you cast chargers first thing in combat before anything moves you'll get the most exp for it. If you do it while only a handful of enemy units is still alive you get next to nothing.