"Guardian I" and "Guardian II". As explained before, the mage simply needs to increase the physical defense of their troops, otherwise battles with the Barbarians will be extremely difficult.
"Charismatic Leader I" and "Charismatic Leader II". Again, about the old – we increase the chance of getting an extra action.
"Destined One I" and "Destined One II". Dealing critical damage is always a good thing. And sometimes, combined with the previous point, it can be a decisive factor in battle.
"Aggressor I" and "Aggressor II". We take them to increase the effectiveness of attacks of kappas, shark guards, and deep hydras. The first two units will overwhelm the enemy with numbers, jumps, and wounds, and the last one will deliver a concentrated fourfold attack, accompanied by a battle cry.
"Ambush". Yes, nagas already have the highest initiative in the game. But still... Sometimes, even those 5 units added to the initiative level can trigger an earlier turn for certain creatures of the Sanctuary.
"Counter Strike I" and "Counter Strike II". Increasing the damage dealt by counterattacks by 13% and the ability to retaliate one more time during the first three turns seems like a good advantage.
"Regeneration" and "Regeneration (General)" / "Healing" and "Healing (General)". I don't see the point in learning both. Experience points are a valuable resource, and spending 4 on just healing... that's not for me. The principle of choice is as follows: if the opponent plays as Haven, take only the magic healing of the Light. In all other cases, go to the Earth magic school.
"Summon Earth Elementals". To be honest, the warriors from this pile of stones are not very good, but their innate regeneration, combined with the "Petrify" spell, compensates for this shortcoming. First, we replenish the ranks of our army. Second, we gain the ability to temporarily remove one of the enemy units from action. Third, we save an experience point.
"Summon Water Elementals". The situation is similar to the previous one: we increase the army; we moisten the enemies, making them more vulnerable to attacks of priestesses; we get "Ice Block"; again, we save an experience point.
"Slippery Ground". Well, they say it's not bad to limit the opponent's mobility. I always choose it, but I haven't actually used it yet.
"Water Magic I", "Water Magic II", and "Water Magic III". For the Sanctuary, this is the foundation. It boosts all abilities based on Water, of which nagas have plenty. In general, we definitely develop it.
"Eternal Winter". A passive characteristic that affects not only the effectiveness of frost magic but also the effectiveness of snow maidens in battle.
"Ring of Cold". It should be in the hero's spellbook as a powerful spell for any occasion. Moreover, it causes target units to freeze – and that's about the previous point.
"Ice Bolt". The "Cloud of Mist" of sacred kirins and the "Ice Block" of Water Elementals automatically make the choice of this ability justified.
"Protection from Magic I", "Protection from Magic II", and "Protection from Magic III". It is very important to have this when facing any faction, except for the Barbarians, whose only units that deal magic damage are the stalkers. Moreover, it would be good to reduce the effectiveness of Air magic used by the enemy.
"Stop Time". An excellent weapon against any enemy, and it's especially pleasant to use it against angels.
"Guardian of the Secret I", "Guardian of the Secret II", and "Guardian of the Secret III". Again, these are necessary skills for the Overlord of the Tides. Half of the Sanctuary army will start hitting harder, and additional magic abilities will become much more powerful. Add to this the full development of Water magic, and you will see what we get. ;)
"Magic Affinity I". I discovered this ability for myself just the other day. And it really isn't a waste of an experience point – increasing the hero's maximum mana by 30 units often makes itself felt towards the end of the battle, when there is exactly a lack of magical energy to win. I recommend it.
To the conclusions of my previous large post, I will add that nagas have many diverse means of neutralizing the enemy, which allows them to first deal with one part of the opponent's army, and then apply the rest without proper resistance. And we must not forget the extreme importance of skillfully moving the sacred kirins across the battlefield: "Cloud of Mist" is the main element of the Sanctuary's strategy.