You're thinking about this the wrong way. Soldarin's method is, whatever you get from your monster HD *replaces* what you would have gotten from character HD, it's not retroactive. You add one level to a 12 HD monster, and you have 13 HD - which gets you nothing. If you add two more levels (3 levels total, 15 HD) you get a new feat, because normal characters get a feat at 15th level. You add another level (4 levels total, 16 HD), you get an ability increase.StealthyMark said:On contrast, if you would (wrongly) add monster HD and class HD together, the archon would receive one feat (1st, 3rd, 6th, 9th, 12th) and three ability increases (4th, 8th, 12th) immidiately upon gaining one single class level.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.