The exception to this would be classes that stacked with each other, such as rogue/barbarian (?, or wizard/sorcerer?, fighter/ranger?) Even then, it sounds risky.
Here is a 12 level fighter/adventurer. A would be companion of Lidda and Hennet. Bolgor is an example of when multiclassing works, and IMO more viable than a straight fighter.
He has decent levels in the skills that I think adventuring would require (climb, jump, spot, listen, hide, move silent and balance). Fights pretty well, has decent AC and HP, strong saves + evasion and better than average land speed. Uncanny dodge and Endurance (which lets him sleep in his armor) keeps his defenses up at most times. Mobility and spring attack lets him deal with opponents with reach.
In a pinch he is able to heal himself with his wand (and possibly use other wands/scrolls with spells from the rangers spell list). He is still not able to make it alone, he needs magical support to deal with the threats you can expect at level 12.
Bolgor CR 12
Male human fighter 5/ranger 3/rogue 4
NG medium humanoid
Init +3; Listen +13, Spot +13
Languages Common
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AC 25, touch 14, flat-footed 25, Dodge feat +1 to AC vs. 1 opponent
HP 94 (12 HD)
Fort +13, Ref +12, Will +10
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Spd 40 ft.
Melee +2 falcion of frost +19/+14/+9 (2d4+11 +1d6 frost/15-20)
Ranged composite (Str +5) longbow +14/+9/+4 (1d8+5/x3)
Base Atk +11; Grp +16
Atk Options power attack, cleave, rapid shot, spring attack
Special Actions Sneak attack +2d6, evasion, improved uncanny dodge, favoured enemy +2 (undead)
Combat Gear 2 potions of cure serious wounds, 2 potions of invisibility, wand of cure light wounds, wand of entangle
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Abilities Str 21, Dex 16, Con 16, Int 10, Wis 12, Cha 8
SQ Human traits, trapfinding
Feats Iron will, Power attack, Track, Cleave, Weapon focus (falcion), Endurance, Dodge, Mobility, Weapon specialist (falcion), Improved critical (falcion), Spring attack
Skills Balance +9, Climb +16, Hide +14, Jump +25, Listen +13, Move Silently +14, Spot +13, Survival +11, Swim +7, Tumble +14
Possessions Combat gear, +3 mithral breastplate, +2 falcion of frost, boots of striding and springing, amulet of health +2, girdle of giant strength +4, gloves of dexterity +2, wand of cure light wounds, wand of entangle, cloak of resistance +3, +1 animated heavy steel shield, ring of protection +1, adventuring gear, composite (Str +5) longbow.