You have much output damage but nothing to absord incoming. Your block Fury is an idea, but since he has no Taunt, it's only chance that he gets hit instead of you or the other rangers.
Of course, we're talking here about legendary/epic bosses, not normal monsters with dexterity incapable of hitting more than once over 3 rounds...
In your case, I would choose the following (I'll talk about Tier 5 'cause there's no point to change your mercs now for 1 level).
1 Guardian with at least 7 levels of Taunt, Shield Mastery and Counterattack and at least with x4 base Vitality. That's your meat shield. Every vitality granded by items will be multiplied with the same multiplier so you need to maximize his Health (it's not difficult to take it up to more than 10k HP). With a high-block shield and a 1-handed sword he'll reach around 100 block, which will be decent even against bosses with ~500 Strength (that's -50 armor). A high Counterattack will also add on the damage output since your Guardian will attack every enemy he blocks, which can be up to 6 bonus attacks per round (if you're lucky enough). Ignore weapon masteries. Blundly put, this guy's main job is to die in your place, so make sure he absorbs as many hits as possible before he falls, instead of dealing 50 more damage.
1 Light Mage with at least 5 levels of Buff Mastery, 7 levels of Fountain of Life and at least one more buff (the higher level the better). Panther Swiftness is a good one too since it raises your group's Agility, thus reducing your enemy's critical rate. Things are simple here. 5 levels of Buff Mastery will cause your whole group to get all your Mage's buffs in the first round. Fountain of Life greatly increases your group's Vitality, thus Health. So if you get only a couple of hits per fight, that bonus Health will absorb the whole damage, letting you in full-HP after the battle. Inner Spirit is also a nice addition since it can increase your merc's health with a significant amount, making him less of a "victim" in case of splash-damage enemies. Ignore Heal skills, they are worthless at Tier 4 and above. After the new skills are implemented, a Light Mage with Ressurection would be a good idea.
1 Dark Mage with at least 5 levels of Curse Mastery, 7 levels of Sickness and at least one more curse (the higher level the better). High level Petrify is a nice addition, since it will cause a random enemy to lose his next move and it is used each round. As with the Light Mage, 5 levels of Curse Mastery will land all curses to all enemies on the first round. Sickness reduces enemies' Vitality, practically causing nice damage on the first round. Poison Cloud is also helpful since it lowers enemies' Agility, letting your heavy armor users hit more frequently and lowering the reduction your enemies cause on your critical chance. Inner Spirit is good here too, same reasons.
1 damage dealer. Since you already are a ranger, I would choose a Fury Warrior here for you with at least 5 levels of Wirlwind, at least x4 Strength and 1 mastery (I'll talk about that later). I don't know if it is possible to find a Fury Warrior mercenary with Battle Cry, but if it is possible it would be great (7 levels of Battle Cry lower all enemy attributes by 24%). Berserker Stance is a nice addition, since it can raise youe merc's damage for up to 30%.
Stay away from Crushing Blow! It lowers your merc's chance to hit by 50%, something you don't want for just +140 attack (at maximum level). Now, regarding masteries. There are two ideal choices in my opinion. Warhammer Mastery and Dual-Wield. The first gives your merc bonus to the weapons with the greater damage, warhammers lower though both Agility and Dexterity by 30%. If you choose this, make sure to include +crit chance to your merc's weapon to make up for the lost dexterity. Dual-Wield will result in a bit less base damage but with 2 +crit swords, you can easily reach 35%+ critical chance with a maximum damage of around 1400 (with x4 Strength it is not difficult to reach 400 Strength, which is basically +100% damage). The Dual-Wield will also offer you around 40 block, which is not much, but you still have chances to block an incoming attack. If you choose a ranger, well... you know rangers better than me I guess.
The fifth one can be either one more damage dealer or one more guardian, based on what seems to be going wrong with your group (e.g. if your tank dies before the enemy boss is at least at half its health, you need a second tank to absorb the rest of the damage, if the same happens, but you are already fighting for 20 rounds or so, you need more damage so you need another damage dealer).
That's more or less my point of view. I would be really interested to a whole other build that would work.
EDIT 1: I remember I have defeated you in Falar at least once and my mercs wear mostly blue items (with only some of the important ones being legendary or epic).
EDIT 2: I just owned the Ice Axe.
Battle saved in first place of favorites, you can see my mercs to get a practical idea of what I discussed above.
