ATM, Im only level 56 but have a fair idea of some tactics you should use in XP groups and in raids--lets start with XP groups.
1. Unless your group is very tank heavy, your group STR, DEX, AGI, etc spells are a bit worthless. Everyone can use more HP and AGI for better AC if they get aggro, so use your group STA and group AGI spells for that. If you have encumbered groupmates ( ask them they may not know it ), then STR buff the group as well. If you cant afford the mana or dont have the spells, use JB buffs or the lower mana single target buffs. If theres no chanter make sure MT has celerity or better ( i know at some point 60+ we get 60% haste spell--if u have that use it--make sure pet has this spell as well and a STR buff--thats all Id put on pet unless soloing ).
Ok to your first question, your best defense is to not get aggroed, easier said than done. WHEN you get aggro, if the mob isnt slowed by then either root it and back up or if you are in danger of dying run small circles around your party to stay in healing range and let them know you need a heal. If the mob IS slowed, then try backing off a bit and the MT ( main tank ) should be able to re-establish aggro fairly quickly. If it wont leave u alone, you may want to try a blind spell ( not reccommended ) or just root it. Use the level 5 root, with everyone wailing on the mob, pet and DoT/DD damage it wont last long but will buy you some time. Never try to heal yourself while an unslowed mob is beating on you, they take too long and will most likely be interrupted and you will die, especially on level 45+ mobs. You should cast Turgurs First unless the mob is really high MR or level 58+, then cast Malo or Malosini ( have both loaded ) depending on what spells you have.
2. If adds come your puller isnt doing the best of job or isnt the rigfht class, but sometimes you dont get theperfect group makeup. Chanter should be main crowd control, mezzing adds. Tigirs Insects is the AoE slow you refer to is only a 50% slow but I believe has a hate reduction factor in the spell too, but if there are enough mobs to use that spell you better be evaccing out of there or your party is trashed. Maybe use it to buy the wiz/dru a few secs to get off a evac. If the adds arent real close to each other or the party yet, root may be the best option, however in that case you want to use your best root. Its best to have a chanter for crowd control as even ONE add in a zone like Plane of Nightmares will rip a caster party a new one quick. We had a full group last night I was the lowest at 56 and MT was a 64 BL. At 61 druids ahve a better 75% CH than we do, but if no druids are there definately have that spell loaded. Note you will almost require a cleric and a enchanter for some of the mobs here.
3. The way I fight in groups like that is I have loaded chloroblast, group regrowth,cannibalize 3 ( or 4 in your case ), buff slot, buff slot, malosi ( or malo or malosini in your case ), root, and Turgurs. Turgurs landed alot more often than malosi did on mobs I was fighting last night and usually this is the case, chanters wont always tash at first as the have to deal with mez aggro and have crap HP--they will ALWAYS want you to slow--as soon as the MT has picked a target, /assist him and slow that mob asap. Mobs usually dont come after you unless you fail to land a slow or 2 then cast again and slow them.
If you can get Malosinia ( -70 to all resists except disease ) use that and Malo to debuff real high level mobs , casting malo first of course.
Dont waste mana with DoTs or DD, just about every other class can out do us there and if you get resists on slow and maloxxx you will eat mana quick if your party is pulling at a good rate. Send in pet once MT has engaged mob and its slowed. After you maloxxx it then JBB blast it if u have one, and cannibalize inbetween JBB blasts to recover mana.
If you dont have any AA yet Id reccomend the following--hp regen 3, mental clarity 3, mass cannibalize, and if you raid alot mass group buff. Also run speed one is good if you get dispelled and need to make it to a zone line ( or switch out your boots for epic boots--instant cast SoW ). Keep regen and greater bearform on, at level 56 I have 25 hp regen/tick self buffed--iksar and troll shaman will have alot more--thats more mana for you. Get an iksar BP of Fungi tunic for more HP regen too if you can afford/find them

Miscellaneous thoughts--always keep a few Kilvas Skine of Flame DS potions on you--they stack with druid or mage DS, cleric reverse DS and bard song too as well as coldain ring DS I think--our MT last night had a 76 hp DS on that gave him 15 hp back every time the mob hit him. Every bit of offense helps and at level 60+ the potions last a long time--like 15-16 minutes and are atleast a 31 hp DS.
Keep an eye on your cleric and especially chanter--they can get aggro fast and an unslowed mob will tear them a new one quick--if they have aggro start casting heals even if their HP arent dropping--they will be very soon very fast.
I make hotkeys for the following : JBB, attack, /pet attack, /pet guard me, /pet back off, %t has been slowed, %t has been malosied, and varios gear that casts effects ( like my cure disease and cure poison earrings--saves me a buff slot). I have several more but those get used the most. Solo, buff the pet with all the best buffs you have, in a group just haste and STR, and in a raid ask if they want you to have a pet up.
When I solo I go to places like SF where JBB as its cold DD almost alwasys hits even unbuffed level 48-52 mobs full damage every time. I stack Bane of Nife and E-bolt and then let mob chase me until pet gets back aggro ( I almost always have pet taunting on ), cause now DoTs do full damage even when mobs are chasing you.
Never use the cripple line unless its an epic fight where that 1% reduction in mob effectiveness may make or break the fight--I never see any difference on short fights to justify the mana.
Big thing for shaman too is know what mobs are unslowable, and what mobs can only be disease-slowed ( Plague of Insects and the new PoP one )--if u have to use them use insideous decay to debuff their DR--its 1.0 sec cast time and unresistable.
Shaman can melee to with the right gear but only in real limited situatuions. I will step in and take hits if my pets at like 10% life and the mob is nearly dead so he will hit me and not kill the pet which is nearly 2000 mana in remaking/rebuffing pet. Our pets can solo light blue mobs at 56 provided the pet is fully buffed and the mob is Turgur's slowed. Throw in a few JBB blasts if you have it and he can solo low blues with no mana loss from you.
Well hope that helps some if you need anything else please post.

56 Ogre Luminary on Luclin Server--Chaotic Winds--Baron Aggrok