Getting Carbuncle (and thus unlocking summoner at the same time) is a matter of getting a very rare drop and travelling the world in search of various weather. If it's difficult, it's only in that unlike other tasks, you're at the mercy of the FFXI random number generators it seems. No matter how high level you are, if Fire Weather isn't being cycled through for example, you aren't getting your Carbuncle. If the Carbuncle Ruby refuses to drop, you're not getting Carbuncle. Getting summoner is more about good luck than great skill.