You gotta understand something about aggro and threat. When you pop heroism, trinket-crit that lightning bolt/chainlightning/healing and go to town, no tank in the universe will be able to outthreat you, unless you have the benefit of blessing of salvation for instance and/or have a pve spec with threat reduction talents. He may spam swipe until his claws are totally gone, it wont change the fact that your TPS (Threat-per-second) is higher than his. If he tanks both the reflect guards with one of your pvp specced shammies spamming weak heals on him, he WILL die sooner or later, or your healing TPS will surpass his TPS and you will die instead.

The trick is to reduce the amount of healing needed as fast as possible, which means to dispose of one guard quick. Pop your heroism and go to town, even use a NS lightning bolt on the first one to save a second dps time. 4-5 LB and a CL later the first one should be pretty dead. Now you can just kill the other one controlled. I for example have enough dps to kill both of them long before they even think about shielding.

I use this macro on one of my guys, all have 40/0/21 spec (you could of course heal with ie. 2 guys, but you'll be hampering your dps too much, and it's not really needed if you use heroism and cooldowns on the harder mobs): /cast [target=targettarget, help] Lesser Healing Wave; Lightning Bolt.

What this macro does: It checks whether or not my target is targeting anyone (unaggroed mobs have no target). If my target isnt targeting anything, it will shoot Lightning bolt at my current target. It could just as well be a simple /cast Lightning Bolt macro. If however my target is targeting something the [help] argument will find out whether or not I can help my targets target in any way. If I can help my targets target, in this macro I choose to cast Lesser healing wave. If I cannot help my targets target, I will still shoot lightning bolt.

Practical usages. I want to heal whoever got aggro, but if noone has aggro I want to utilize the full dps potential of my group. For me, or for you for that matter, it doesnt really matter much if one of your shammies has to tank something for a few seconds. You still probably got close to 10k armor so you'll not go down in 1 swing. And since your druid tank got all kinds of nice things to pull outta his pocket, you'll not be in danger for long. A single lesser healing wave or two will probably save you until your friend has taunted or stunned the mob or whatever.

Typically I start a fight with 2 lightning bolts (my target hasn't targeted anything until about the time the 2nd LB lands. With heroism I can cast 3 LB's before my target targets something), then I continue with a CL, before going back to LB spam. Now however, my target is angry at whomever critted the most, so I'll need to heal that target. So the shammy with the /cast [target=targettarget, help] Lesser Healing Wave; Lightning Bolt macro will now heal whomever the mob targets. If for example I pull multiple mobs and some of them start attacking searing totems, and I decide to nuke this target, the macro will fire lightning bolts since you cant [help] a totem in any way.

Just slot that macro in instead of your normal lightning bolt spam button for any of your guys. I use my main so I can quickly abort unneeded heals before they're cast to save mana, and to retain the ability to pot mana pots when needed, and yeah - they are needed when your spamhealing on heroism/bloodlust vs some of the bosses with lots of hitpoints. Warchief Kargath for example. Be amazed, it worked wonders for me anyway. Good Luck!

Edit: Just a couple of things more. This will naturally heal the shit outta your Earth Elementals as well, and make them last alot longer vs the tougher bosses. Another benefit of this nuke tactics is that your tank will have time to get ahead of you on threat on the mobs that you arent nuking. In the case of the reflect guards in SP, your tank will probably need to help you dps down the first target if your dps with heroism/cooldowns still isnt good enough to finish the mob.