I'd say that if you haven't yet done the heroic, get a couple of lvl 30s to help you complete it. Doing it on the mid setting will get you 6 strange coins (hardest gets you 9).
If you've already done that then grind through the Roc strikes, you'll get blue engrams which in turn have an ok chance of turning into strange coins.
I went for Obsidian myself, but that's because I have another awesome sniper rifle (Blackhammer).
I'm probably pushing my luck that Icebreaker will be sold again or that I'll get it as a randomised* reward, but I'm ok with that :)
*not earned ;)
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I'm grinding Roc strikes right now. Hopefully I'll get enough to get both before I start my first raid.