Well, "reliably fighting armies solo" is not mundane stuff in Ars Magica; it's supernatural stuff, and of fairly tall order.
But... how about a character like the following? She's a bit of a one-trick pony (she's supernaturally good at swordfights, but that's it); still there's a lot of interesting tactical stuff she can pull-off in a swordfight. She's designed as a standard Companion, with 9 points of Virtues balanced by 9 of Flaws, so there's still room for customization (particularly if you want to go the Mythic Companion route).
Yvonne, outlaw and former faerie squire
"My father was a faerie knight - so my mother tells me - and I will become a knight myself when I grow up!". Yvonne was probably the only person in her village who believed it; it is not easy for the lovely daughter of an unmarried washerwoman to find a man (not to mention a nobleborn knight) willing to train her in the harsh arts of war. To be fair, after running away from home at thirteen and narrowly escaping bandits, rapists, and other unsavoury types, Yvonne did find brief employment in a coven of witches and warlocks as one of the "grogs" - the thuggish armed henchmen of the sorcerers. But a thug is no knight, and (after stirring some trouble at the covenant) Yvonne left to seek her destiny. She went through many more adventures, and eventually met a fairy lord who took her as his squire. According to her mentor her learning was incredibly fast: after less than twenty years, he confided to her that she was already showing "remarkable progress" in her swordsmanship, and they would probably be able to move on to her lance in a few more decades. Then mace, riding, history of warfare, etiquette etc. etc.; she would probably earn her knighthood in a few brief centuries! Yvonne fled back to the mortal world. Yet she found she no longer fit in. After the wonders of the Court of Jasper and Opal, no mortal place felt like home. She roamed the land seeking employment as a mercenary; men laughed at her when she claimed to be a knight (well, she was technically still a squire, but...) . And after she challenged Count Lefevre's men to combat to prove her worth, and slew seven of them while setting five more to flight, she was declared a witch and an outlaw. Where will she go now?
Characteristics: Int+1 Per +1 Pre+2 Com -3 Str+1 Sta +1 Dex+2 Qik +2
Size: 0
Age: 36(21)
Warping Score: 3(0)
Confidence Score: 1(3)
Virtues and Flaws+: Outlaw, Enchantment*, Strong Faerie Blood++, Affinity with Single Weapon, Grant*, Ware*, Envied Beauty**, Faerie Upbringing*, Oversensitive(to desrespect), Rolling Stone***
- From RoP:F.
** From HoH:S.
*** From Grogs.
- Note that Yvonne gains no Flaws from her Warping score of 3 - she undergoes Faerie Calling instead (see RoP:F).
++ Yvonne's faerie blood provides her with a positive Sympathy in swordfights at a basic score of 1, which she raised 7 points further (3 from Warping, 3 from Strong Faerie Blood and 1 from Faerie Upbringing) with experience.
Personality Traits: Touchy +3, Knightly +1, Flirtatious -2
Reputations: Witch 2
Combat: Init +3, Atk +23(+27*), Def +20(+22*), Dmg +7
*When Enchanted with Puissant Weapon, Improved Dexterity, Great Dexterity, see below
Soak: +6
Fatigue Levels:
Wound Penalties:
Abilities:
Athletics 2 (grace), Awareness 1 (alertness), Brawl 1 (dodging), Enchantment 4, Etiquette 2 (Faeries), Faerie Lore 3 (faerie courts), Faerie Sympathy (Swordfights) 8, Grant 2, Leadership 1 (battle), Riding 3 (battle), Second Sight 2 (battle), Single Weapon 9 (swords), Speak Own Language 5, Survival 1 (local area), Ware 2.
Equipment: Mail Haubergeon, Coif, Longsword (Load 3, Encumbrance 1)
Note: Yvonne was mentored to wield the sword at a level of ability unheard of in mortal lands. Not only her Characteristic+Single Weapon (including her Sympathy as a specialty) is a phenomenal 19; her mentor has taught her a number of secret techniques that she can bring out with Enchantment (using Single Weapon as the performance ability) in combination with Grant and Ware. Her Sympathy score of 8 allows her to perform Rites of up to level 40; although her Method+Power total is lowish, she can build up impressive casting totals over a few rounds with Enchantment (see RoP:F). Often, she performs these Rites while practicing, setting them to affect her on her next real combat (or her next sparring session) with D:Geas/While in battle (+3) or D:Geas/Sun (+4). Typical effects include Granting herself Minor Virtues like Puissant Weapon, Cautious with Single Weapon, Improved Dexterity, Great Dexterity, and a Minor Blessing that always makes a sword available when she needs it if it's remotely possible (Base 15); Warding herself from any mundane opponent with whom she's involved in a swordfight (Base 1) or from any with whom she's not, as long as she is in some swordfight (Base 15); Granting minor (Base 10) or Major (Base 30) flaws on her enemies while she fights them with her sword; slicing through, or parrying, supernatural effects with her sword etc. She can literally fight mundane armies single-handedly.
EDIT: in the original post I had mistakenly listed Yvonne's "starting" score in her Sympathy at 4 (1+Warping score). As correctly pointed out by callen, according to RoP:F it should be 1. I fixed that, and balanced the 45xp I had thus overspent by reducing from 3 to 2 Yvonne's Etiquette, Grant, and Second Sight scores.