My Behemoth cannot tank Alerts unless someone on my team tosses me heals. Period. Alerts do not even guarantee that there's a Support player in the group at all, let alone someone who knows they'll be expected to heal.
I feel for you man. I know your pain. I played a Behemoth AT, before deleting it around level 25 because it just was a terrible tank. Defiance is NO GOOD unless it's backed up by a self-heal, damage shield, damage reduction, or some other form of mitigation, and the Behemoth just doesn't have one. It's, honestly, one of the worst tank ATs for that very reason. It's why I switched over to a Glacier AT for my tank and never looked back.
Behemoth is a really bad example of a tank, to be quite frank. Any fight that requires you to tank more than 1 or 2 enemies at a time is going to leave you hurting, and Defiance in particular is slow-building. Sure, once you're sitting on eight stacks you're about as tough as nails, but you actually have to
get eight stacks running at once, which can be difficult. You're also stuck with tons of powers that cause knockback, as well as Enrage which
requires knocks to stack. This means you're usually running around the battlefield like a chicken with your head cut off, unable to properly AoE your enemies because you're scattering them around. Further you can't use crowd control to mitigate damage, because you have to take hits to build up Defiance, which is terrible in the absence of a self-heal (and Enrage's self-healing advantage is pitifully weak even in the best of conditions and scales with Presence which the Behemoth doesn't have much of; I don't even count it).
Basically, what I'm saying is that your perception of tanking in CO is being colored by a terrible AT. And I know being silver doesn't give you a lot of options (I'm a silver player myself, and c'mon, only two tank ATs for silver players, but three ranged damagers? What gives, Cryptic?), but, y'know this is how they get you to subscribe to Gold. Or how they try to, anyway. I personally wouldn't give my prospective customers outright disappointing builds like the Behemoth, telling them they have to subscribe to see a good one (when if they subscribe, they could freeform a build better than any I'm willing to offer ANYWAY.)
Short version, the Behemoth is terrible. I don't know why they don't rework it, why it never performs as-advertised, but it doesn't. It's a failure of game design on their part and I will freely state such to whoever is thinking of trying it. It MIGHT pan out at high levels, but I don't feel like playing a character that has to wait until level 35 just to be capable at its advertised role. It's the same reason I don't play The Mind (which is another AT that is suffering defined until you hit the level cap and get your last power.)
My advice is to try coming up with a character for a Glacier AT. Try that out, and if you find you're still despising tanking in CO, then maybe it's just not a good game for you. If you do like it, then consider subbing so that you can Freeform up a Might character that can stomp the daylights outta your enemies in the fashion you desire.
EDIT: Hey, if any Cryptic devs ever read this and want to rework the Behemoth, here's what I'd recommend to make it actually good.
Lv1:
Clobber,
Iron LariatLv6:
DefianceLv8:
ConvictionLv11:
Iron Cyclone OR
RoomsweeperLv14:
EnrageLv17:
DemolishLv21:
RetaliationLv25:
AggressorLv30:
ResurgenceLv35:
Havoc Stomp OR
ThunderclapLv40:
ShockwaveIron Lariat gives you a means of pulling enemies toward you if you happen to knock them away, and neutralizes ranged enemies who you'd normally have to walk toward. It also forces ranged enemies to fight you with their melee attacks, which are weaker and easier to survive. Getting Conviction early gives you a self-heal to work with, and Resurgence is another one that's useful. Iron Cyclone is one of the best Might AoEs for tanking purposes: with the advantage, everything that gets caught in it gets knocked TOWARD you with every tick you maintain it, proccing your Enrage and neatly gathering up a scattered group up enemies for further AoE abuse.
Havoc Stomp gives you a Tier 3 AoE prior to level 40. When you have Iron Cyclone and Havoc Stomp, you can abuse enemies relentlessly with AoE knocks. Iron Cyclone pulls them in, Havoc stomp sends them skyward, and you can repeat the combo until you run out of enemies, at which point you unleash a fully charged Demolish with 8 stacks of Enrage on any survivors.
If I could, I'd replace Defiance with either Invulnerability or Regeneration, but other ATs already use those two, and The Behemoth is the only AT that uses Defiance.