Get a Bad Juju to go with your Obsidian Mind, you'll be dropping nova bombs like mad. Also, don't be fooled in to thinking sun singer is PvE only and vice versa for void walker. If you have max discipline on sun singer, you can be extremely deadly with radiance in the crucible. Void walker can be great for PvP too, but with the options that the melee gives you it is better for PvE IMO, like speed boost for the thralls in CE. Playstyle wise, Warlocks are a little squishy compared to Titans, but have much better battle recovery, so try to always know where you can find cover.