Stockades - Max it. It’s useful.
Conceal - I’d say put 1 skill point and that’s all, especially since your traps usually don’t last that long. (unless you’re going PvP, then that’s another issue)
Claymore - Damage scales quite well until lv 8 or so. I’ve not used it much just because the game is so laggy that most of the time, I’d be near death before Claymore kills any mobs.
Punji - Unless you’re going on a kneeling shot build or heavily PvP, I’d say 1 is more than enough since to set it up, you need to charge it.
Detonate traps is useful when you want to be an as*hole Sapper and detonate your team mates’ Safety Zone, heals etc, or when you have no more mats for your traps. Slap the two Stockades on the boss then detonate away. It does detonate the bosses’ traps but you have to be very very close to it.
Collar Bomb - It’s useful, now that I’ve finally found the trick to it. Go melee while using this skill and aim for flying or small mobs since those are more likely to trigger the bomb. Dump all remaining points here. Might help a little more than upping some of the other skills (Punji/Conceal/Claymore).