i think the only way to beat that shaman is to "scare" him. do as much damage as u can before it gets to melee range, so for a lock a searing pain spam is better than the DOT's forst because u waste 3 GCD's before u start ur real burst and ur DOT's will also only start ticking after 2 GCD's. if u can either get him low mana/health OR let him cast because he realizes that you will keep the burst on you that way, he'll waste even more mana. dont do DOT's before ur searing pain is silenced. and use his healing casts to run away a little bit(even if he has EB totem up) if he can not melee, he will have to either burn mana on casts or walk to you getting hit more and not able to heal himself on that situation. there is no presure as far as i kno if the shaman is at low HP. even if he stays at low HP, u can fake cast fear a little before he is close enough to you. than fully cast fear when he is close to you. 70% chance that he will not interupt it because u can not afford to fail for a fakecast when u got a lot of presure on your HP/mana. ofcourse he will pop AGM and stuff after a while, but this is, as said before, only a way to possibly kill it. i think it is the best way to do it like this and it did work pretty well so far for me.
as you can see, if u actually wasted your time to read this, the other weak point of the smaman is that he has to melee to do decent damage otherwise he will run OOM way too fast leaving his back to his AWSOME ability to heal himself. therefor i even think that a melee class will have to stay away from him and force the shaman to cast and LoS/bandage urself when the DOT's are gone. a warrior or rogue can force him to even pay a little attention to his mana because they can quickly attack the shaman and CC for some free distance to force him casting.
most inportant thing to a shaman: it is actually good not to LoS against him when he tries to make a cast. unless u can not bandage urself(or cast a healing spell) because it will waste way too much of his mana when he manages to finish his lightning bolt