I guess the skill itself isn't deemed very hard to train it is pretty quick to level, I'd say most of the effort comes in collecting the charms. It really depends on a number of factors;
Do you already have charms?
Do you enjoy monster killing? (This is the main way to get charms)
Once you obtain the charms it's relatively straight forward and easy, although it can be a pretty expensive skill to level. So like I outlined above, the main part which makes summoning a harder skill to train is in the collecting of the charms, however there are processes which you can use in order to get more charms for example bursting rock lobsters.
You can also use the familiarisation distraction and diversion which provides you with triple charms, for up to the next forty minutes of in-game time, to any charm drops that you receive.