And here's Part 2 of 2 (got distracted)
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Makotolia: 1. both sorcery nodes contained 3 units of phantom warriors each, and as you said, they are the easiest, though I got Brax killed repeatedly until I realized that I can just summon as many phantom warriors myself to kamikaze them XD
Something to keep in mind about Phantom Warriors: They have no defense, so they fall apart like a house of cards as soon as a stiff breeze hits them. However, their Illusionary Attacks ignore the defense of whatever they are attacking. They cut Brax to pieces because he takes every bit of damage they dish out (up to 30 damage in a Sorcery node!). If a target is immune to Illusions, then the illusionary attack ability has no effect and the unit's defense applies as normal (the phantom warrior still gets their normal attack, they just don't bypass the defense of a unit that is immune to illusions). Don't send Phantom Warriors against undead units; the phantoms will be cut to pieces without having any effect.
Something else to remember: any creatures within the influence of a node (like in the node itself) that match the node's type receive +2 to their melee attack, all ranged attacks, defense, and resistance. Phantom Warriors are Sorcery creatures, so they get +2 attack, +2 defense, and +2 resistance while fighting within a Sorcery node. This bonus applies to attackers and defenders, so your Phantom Warriors get that same bonus. HOWEVER, if a unit is summoned mid-battle, they don't get the bonus immediately; it will be applied starting on the next round the unit is in the battle. So don't summon your Phantom Warriors within a Sorcery Node and immediately attack with them; let them be in combat for a round so they have those extra stats. Hell Hounds are stronger in Chaos Nodes (their fire breath also gets a +2), and Nature creatures are stronger in Nature Nodes.
Yet another thing to remember: the stats that are displayed on the creature stats screen are the statistics that each figure in that unit has. A unit of Phantom Warriors has 6 figures when at full strength, so when it attacks it will throw 6 swings at an enemy (one per figure) where each swing is strength 3. Each PW has 1 hp, so if the unit takes 3 damage, it will lose 3 figures, and it will then only throw 3 swings (each at strength 3) when it attacks. A Holy Armor spell will give each figure within a unit +2 defense; buffs and debuffs have a bigger effect when the target has more figures. On the flip side, a unit with more figures loses combat effectiveness much faster when it starts taking damage, while a single figure unit will fight at full power until it is dead.
Makotolia: 3. the other nature node has the earth element I mentioned as its guardian, and Brax always got one-turn-killed by it. I read from wikia that it may be better to take it down with units with ranged attacks, but I haven't investigated much in building units, so... and the normal bowmen of high men seem quite weak to me, with their strength 1 ranged attack. Now I'm a bit lost, as I don't know if there's a way to tackle this node now, or I'll have to build up my army first, and if it's the latter case, what units should I built? Btw, Merlin's capital is populated by gnolls.
Earth Elementals have 25 attack and +10% hit chance before counting the Nature Node bonuses. That is a lot of damage thrown at anything the Earth Elemental attacks. Each sword icon has a base 30% chance to hit, so the Earth Elemental's hit bonus gives it 25 x .4 = 10 damage on average per attack (again, pre-node). Your Brax probably has only 5 or 6 shield icons, which each represent a 30% chance to block a point of damage, so he blocks about 2 damage per attack on average. All unblocked hits go straight to his HP total.
You'll need to swarm the Earth Elemental with units that have high attack values, and you'll need to expect most of those units to die in the process. Gnolls are actually really good for this, since they have an innate +2 attack for their units. If you have Heroism and Holy Weapon, then you could possibly build a stack of 9 Gnoll swordsmen, enchant all of them with both spells, and send them in. They might be able to deal enough damage to kill the Earth Elemental before being wiped out. A stack of 5 or 6 buffed High Men pikemen should be able to kill the Earth Elemental, though again most of them will probably die. 9 pikemen is expensive, but they'll certainly be able to get the job done. 5-6 buffed Gnoll Wolf Riders should be able to get the job done, and they might have enough durability to survive a single exchange with the Earth Elemental; swing once then back up and let someone else take a swing. Feel free to mix gnolls and high men together in your attack; any units you control have no problems mixing with any other units you control.
Alternatively, ranged attacks can bring it down if you have enough fire power. Bowmen only have 1 ranged strength at their lowest experience level, but they'll get better with some levels (Heroism to the rescue again!). I'd probably just build the Gnoll wolf riders, since surviving wolf riders are still excellent to send on other attacks.
Makotolia: 4. the chaos node is guarded by 4 units of hellhounds. After having got Brax burned down to ashes a couple of times, I'm not sure what should I do now.
Buffed Gnoll infantry or Wolf Riders will be sufficient. Make sure that you initiate the attacks as often as possible, since Fire Breath is only used when the possessor initiates an attack.
Makotolia: I also conquered some towers so far (not wizardry tower, just ordinary ones), most of which are occupied by undead creatures, and I researched and learned the life spell Star fire to shower them with. I feel that as an Archmage, this is a quite Horus way to do things, especially now that I have some power to further increase his casting skill.
Archmage is an excellent wizard ability. Make sure that you are spending as much power as you can spare on the Skill slider (in the Magic screen) to increase your casting ability; Archmage provides a bonus to all power you spend on increasing your casting skill.
Makotolia: Now my magic spirit just found another continent. I don't want to leave Brax idle at home, but I'm not quite sure how to conquer the remaining nodes.
Build an army of normal units. :)
Gnolls are an aggressive race. They can't build many buildings in their towns, but all their units hit pretty hard. If you only have one Gnoll city, then I'd suggest sending out one or two Gnoll settlers to found some extra cities. 2-3 Gnoll cities are more than enough to generate a flood of Wolf Riders. Assuming you only have access to Gnolls and High Men, then fill the rest of your continent with High Men cities. Boats and Floating Islands can carry your wolf riders to other continents. While the Gnolls attack, the High Men can be building infrastructure to support your empire and eventually build end game units (like paladins) to pick up the slack when the Wolf Riders falter.