Defeat crota and atheon on both difficulties normal & hard mode. Hit the exotic chest in vog on normal & hard mode. Normal mode first and hard mode last . Beat the nightfall. If you have three levelled up guardians then this strategy alone will grant you a total of 24 solid chances at exotics (gjallarhorn) per week. Running atheon or crota on normal is like a free nightfall mission, you get damn near the same chances at an exotic. I just did crota on normal and got the hawkmoon. You can also try hitting the vanguard ROC strikes and crucible for more slimmer chances at exotics. Also, good luck with those heavy legendary engrams, or any engram for that matter. Hope this helps. Thanks :)