So this is why tanks can't tank?

I haven't played through merciless but I'm not having trouble at all in cruel. Titanicus Span or w/e the shield I have makes ranged attacks do essentially no damage. Elemental resists capped at 80% so not much damage from those sources either. Big life pool with % regen and 7 Endurance charges will keep me from taken any large amounts of damage. Life per hit on ground slam means large groups are even less effective. Molten Shell being one of my highest source of damage I don't really kill things quickly but it is nearly impossible for this character to die even with 4 stacks of poison, health potions keep me up. I'll most likely get chaos resist on gear before merciless. So far no problems though.

I can't tank too well in merc. Yellows can kill me in 3 hits. I do conversion trap and spread fireball totem to soften a mob before I charge in.

you cant really tank tank huge blue mobs in this game, but face tanking bosses and yellow 1 v 1 is possible with acrobatic evasion..
1 v 5 tanking without any yellow mobs is also possible merciless..

the bigger problem is that most melee skill are rather weak compared to aoe spells or ranged abilities.
melee skills are supposed to hit 3x harder than any ranged abilities