Heavy shot is garbage, but then, that's an opinion. I have it, and I prefer to use light shot, because the amount of stamina and time it takes to get off heavy shot is the same as about 2-3 light shots. It balances out.
I have multishot and -enimity. Both are absolutely essential. After that, save your guild marks for the 10 Archer "merit point" passive traits. It stacks to 10, and costs 12,000 each. So, you need 120,000 marks for level 40. However, you'll be the best damn Archer you can be at lv40 :)
Heavy Shot is a good skill, it deals about the same damage as a CRITICAL light shot, without criting(and have the TP drain effect as well)...its not pure awesomeness, but it's worth buying/using(situational as stated in above posts, and you can keep arrows nocked).
About the "merit points" traits, it's not stackable(as of now), you can buy every trait just ONCE. You just get +10 archer affinity.
Buy MultiShot, Out of Sight, Heavy Shot and Affinity in order.
You are going to unlock those(MS buyable or R20, Out of Sight R20, HeavyShot R30 not sure, and Affinity Trait R36*), so you have plenty of time to gather up marks.(And still, Heavy Shot is OPT Rank 40 and Affinity is OPT Rank 50)
After you're done with those, you can buy the remaining traits to use on your DoM, if any.
* - After completing class quest, on respective ranks.