Summoners have no limitations to their advancement except experience. The ability to call, at first, lesser terrestrial spirits is where every Summoner starts, eventually moving higher and farther than before with each progressive summon. Some focus on a specific type of earthly spirit, and some just summon whatever is nearby — either method is acceptable. The power of the Summoner lies in a mental purity that can not be matched; the ability to shield one’s mind by constant self-discipline and concentration on mental stamina/ability to concentrate unbroken is key. After a short while of experimentation, different types of entities, be they angelic, demonic, Olympic or otherwise, can be summoned with little or no trouble at any given moment.
To perfect the art of spiritual synergy is a draining one, but without question Summoners can still hold their own as one of the most powerful types of caster available.