I use a "Fist of Havoc" to counter them in Trials and Invective is very effective in bringing them down also. As a Titan main the Sunbreaker is a bit OP but so is storm caller with some perks. I do say some people are very skilled outside of using Sunbreaker but have noticed a lot of folks who use it are horrible with their combat skills (gun skills more specifically). I do think the Titan in terms of the other classes takes a great deal of skill but I will say everything else about Sunbreaker is weak outside of its super. It's difficult for Bungie because they know this and must find a way to balance it better only in PVP. I think in PVE it's always a positive to have them on my side