With a Single Spell: Mage Armor

Baseline Spell: Mage Armor

In the DungeonVerse Mage Armor is a relatively common spell that increases the wearer's armor class as if they were wearing a chainmail hauberk. Lower-level adventurers will dabble with Mage Armor but eventually abandon it as they gain access to magical armor. A few will keep it since it has no material cost, a low mana cost, and it doesn't create a suit of armor around them, just boosts their AC. This is popular for multiclass characters, especially rogues, who don't like to be hindered by heavy and noisy armor.

Adventurering Potential

Mage Armor is useful to weak and low-level characters, again with low mana cost and no material component

Mages like it as young mages still tend to get into melee range and have to fight foes up close. Their spell arsenal isn't devastating yet, and being unable to equip serious armor and spellcast is a major weakness. Many an acolyte, initiate, and apprentice magic user has used Mage Armor to survive skirmishes with vermin, goblins, and bandits shooting arrows.

Rogues also benefit from the spell at low levels, as having magic armor equivalent to chainmail with no penalty to their sneak and hide skills is a great boon. Rogues at this level are also working to backstab goblins and cheer when they one-shot an orc. Bards also enjoy mage armor for the same general reason.

Nobles and the wealthy will have access to this spell, considering it superior to donning actual armor. Why wear chain when a spell does the same and doesn't crush a man's doublet or cravat?

Spell Advancement

One aspect of spellcraft that video games and anime embrace but TTRPGs neglect is the growth and development of spells. Wizards, mages, and isekai protagonists will pick a single skill or spell and run it beyond its ultimate form, hit it up with buffs, elemental aspects, ridiculous sideways changes, and turn everything on its head. Personally I have read LitRPGs where a character abused and broke the Inventory spell/system and used it to drain a lake, and then dump the lake on a different foe, to watching a kid start eating dungeon monsters with his swiss army knife tech skill and then end up summoning anti-material and anti-armor space weapons to fight dragons, fundamentally using the same skill.

Heavy Mage Armor - upgrades protection to match full plate

Punishing Mage Armor - deals melee damage back against foes that attack it, typically a low to modest amount, 10-20%

Celestial Mage Armor - becomes visible as gold and alabaster armor, increased effectiveness against evil, dark, undead, etc

Elemental Mage Armor - adds an elemental subtype and gains an aura matching the element. Fire Mage Armor protects against fire at a higher efficiency rating, for example.

Elemental II Mage Armor - inflicts elemental status against attackers. Frozen Mage Armor causes cold damage against melee attacks, as an example.

Steadfast Mage Armor - prevents knock down or knock back effects

Featherfall Mage Armor - prevents fall damage

Magic Mage Armor - also adds resistance to magic attacks

Chameleon Mage Armor - Increases sneak chance

Adaptive Mage Armor - allows the user to switch between different versions of the spell within the same casting,

Prismatic Mage Armor

Stat-Boosting Mage Armor

etc

This is not intended to be an exhaustive listing.

Spell Mastery

Spell Mastery is about increasing the base function of a spell, transitioning from Mage Armor to Mage Armor II, Mage Armor III, etc. As mastery over the spell increases, the potency (and cost) of the spell increase. This is a rarely if ever taken path as mastering a spell takes time, regular investments of mana, and rigorous use of the spell itself. Unlocking Mage Armor II might require 100 castings of regular Mage Armor, taking X number of hits, and investing several hundred mana into increasing self-mastery and understanding of the spell. This is the sitting under the waterfall for days on end and meditating on the water ponding down from above.

One of my favorite tropes in fantasy and fiction is Spending Time in the Wilderness. In this trope, the character is isolated from companionship, supplies, and comfort and is forced to rely on their own skills, and in the challenge of surviving this, they come closer to understanding who they are, and gain a better understanding of their abilities. An apprentice magic user can cast Mage Armor on themselves a hundred times across the span of a school year, sitting comfortable in class, studying on the grassy greenspace of the school, and going through the juvenile dynamics of lunchtime seatings, cliques, and fumbling attempts at relationships and gain little to nothing on the spell other than casting it quickly and easily. In comparison, the same student, accidentally teleported into the wilds near a dungeon might find Mage Armor the only thing keeping them alive as they have to set snares for their own food, face regular animals, low level monsters, and then struggle for their own life against a band of hungry goblins stalking through the woods. The first apprentice can cast Mage Armor, good job. The second student spent the time in reflection and used the spell many times in real situations, took injuries and survived, and came away with Chameleon Mage Armor and Mage Armor II.

Mage Armor II - obviously increases the protection level, matching full plate at the same mana cost and duration.

Mage Armor III - moves protection level from mundane armor into fantastic armor, dragon's scale, manticore hide, etc. At this level, small weapons deal little to no damage, daggers and such are useless, and arrows have a 50% chance to shatter on a successful hit and do no damage.

Mage Armor IV - basic immunity to mundane weapons, high resistance to magic

Mage Armor V - godlike immunity to damage

Twists

Power should be visible, as a magic user advances their Mage Armor Spell, it becomes visible. Activating the spell is akin to the magical girl transformation sequence of Sailor Moon or Saint Seiya. At low level, the spell is very demure. As it rises in power it becomes increasingly flashy and obvious. The hero with Mage Armor I is unnoticeable, cruising by in his casual adventuring clothing. The champion with Mage Armor III casts the spell and is encased in a glowing suit of armor that reflects their heart, their culture, their soul. The elfin swordsman is suddenly clad in magical rainment resplendent and reminiscent of Gil-Galad as he dueled Sauron. The Orc-warcaster is garbed in layers of luminous metal plate, animal pelts, and fetish trophies and tokens akin to Warhammer Fantasy.

Increasing the level of Mage Armor has personal costs, as the spell grows the caster become unable to wear regular or mundane armor.

Examples

The Guyver: Bioboosted Armor

The MC gets an alien artifact that lets him don a suit of superpowered alien armor. Replace alien with magic, and you've got a take on weaponized mage armor

Sailor Moon

Magical girl transformation is peak Mage Armor I

The many Mighty Morphin Power Ranger series

The rangers start as humans, then can morph into morph suits to fight, aka visible mage armor. Then they can power up to armored morph suits for Mage Armor II, and dont really need to get into the Zords and MegaZords, that's a different genre. Also applies to Kamen Rider, Mystical Knights of Tir na Nog, Big Bad Beetle Borgs, and the rest of the Super Sentai genre. 

Saint Seiya

Granted by a magic item, the main character can summon magical zodiac-themed gold armor. This is some Mage Armor III stuff.