Poll: Preferences when choosing skills in Heroes of Might and Magic IV.

You need to hide the Mage behind others, so he can't be hit by the first shot.

You can't hide from everyone, for example, from a Devil or an Order Mage with teleportation. And Chaos can hit him with Hellfire, for example. In general, a Mage with Warfare gets good bonuses, amulets, and immunities. Although this isn't for everyone.

Added 7 minutes later
Regarding the Guardian Robe: it's a good item, but 1) it's not always available, and 2) it only works for the first three hits. So, for example, if they attack a Mage wearing this robe, after first checking his strength, you can split up the archers, and it will be great against Elves. In general, there are many tricks here. BUT even if the Mage is level 20 and has Warfare, he won't last long unless there's a strong and slow infantry unit nearby.

Added 11 minutes later

In my opinion, it's better to complement schools of magic. For example, Nature doesn't have direct damage spells, but Chaos does. Nature + Chaos = a decent number of blessings, meaning you can manage without Life (although it has excellent blessings). But you'll get few curses, so you can learn a little Death magic. A Mage only needs Warfare among non-magical skills, although Bob lived without it. On the contrary, magic isn't necessary for a warrior; instead, they should mainly develop combat skills, but you still need to think about what to develop and what can wait.

Nature + Chaos certainly sounds strong and intimidating, but... a simple example, a tournament game. You're playing as Nature. I'm almost 100% sure that, regardless of the spells, the fifth level of Nature magic will be very useful, especially if it includes "Dragon Strength" or "Summon Mantis," then the question arises: to get the most out of Chaos, you need to level it up to 3-4, which requires a lot of experience, and you also need Warfare, or you have two heroes, a Mage and a warrior, then the experience is divided between them...
regarding Death, in my opinion, Death is only needed for Nature because of Demonology, although it's unlikely to reach Devils, but Cerberuses and Demons will do, but why does Nature need Death curses, especially since some of Death's spells are mainly suitable for Death itself, such as "Horrific Song," "Life Drain," "Plague"... so there aren't always common curses