Year 1 was a rough year for the sunsinger in the crucible (outside of ToO). It was almost always better to choose Voidwalker. But with the addition of the destructive force that is the Sunbreaker Titans, Sunsinger Warlocks have emerged as a hard counter.
Radiant skin allows the Warlock to tank a direct hammer, or a few nearby ones.
Glide allows for long air-time, making it hard for the Titan to hit their throw.
And any combination of two Fusion Grenades or melee attacks will shut down the Sunbreaker for good. This is compounded by the possibility for the Warlock to have two grenades and/or melees.
So, Warlocks, when you hear that [i]clink[/i] off in the distance, don't fear. Pop Radiance and run towards it. Rush your prey and don't worry when the first hammer hits you. Pepper them with melee attacks and sticky grenades and they should be dead before they can throw their next one.
If you find yourself on the defensive then float around a bit, take advantage of your high recovery.
Personally, I'm a mediocre crucible player but I've been able to consistently kill two Sunbreakers in a single Radiance (most often in mayhem mode) and still have time for a nice spree.
Edit: Added Tag
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Personally, I feel as though Striker, Gold gun and Void walker are much better counters since they can one hit KO Sun breakers. But hey, you do you, am I right?