Combat Skills are an integral part of Arigon if you're interested in entering the worlds of PvM or PvP. Even if you're a skiller, it can be beneficial to prestige these skills a few times to benefit from the prestige points and cash it provides, and when you've prestiged you can leave the skills at 1 if you so wish!
There are 8 skills in Arigon which contribute to your combat level, and are as such counted as Combat skills for the purposes of XP modifiers & general use. Constitution is the skill which denotes how much HP your character will have, and you gain experience in this every time you deal damage to an enemy - so there isn't a specific guide for this skill like there is with the others.