Here's how ya do it...
Ya leave the biped adventure multi-classing alone.
Ya bring in Dragon Adventure multiclassing.
Argueably dragon as they exist now have spells and abilities bipeds would need 3 to 4 adventure classes to acquire the equivalent biped abilities. Dragons have melee skill, biped warrior equates. Dragons have Breezes, biped life and nature schools equate. Dragons have some damaging offensive spells whose descriptions could equate to something a biped arcane caster might use. Dragons have some debuffs, something that a biped Spiritist or Shaman might use.
Give the 100 level Ancient Dragons the option to become multiclassed in any or all of the above mentioned Dragon equivalent schools. Grandfather 100 levels of each if the dragon wishes. If the dragon doesnt wish to become multiclassed, it loses the abilities associated with the school. Adjust adventure rating accordingly. Furthermore, bring in as many other Dragon equivalent adventure schools as can be logically derived from the list of schools currently available to bipeds.
Leave all cannon-fodder critters alone, i.e. wolves, beetles, fyakki, golems, treants etc. Starting at Tier 4 multiclass the WA critters similarly to PC multiclassed characters, with at least a minumum of 2 100 level schools. Tier 5 do the same with at least 4 100 level schools. Tier 6, give em minimum 6 100 level schools.
Now, everyone can multiclass or not, dragons and bipeds. Everyone who does multiclass gets to enjoy that wonderful little adventure rating. And you now have opponents in the field, every bit as good as you might be, possibly better.
*Casts Flame Ward V on self, plus Flame Resistance V, Group Flame Defense II, dons Flame Resistance teched jewelry, unlocks Flamethrower Locker* Knock yerselves out.