That's why titans only come in 'Massive.' Big enough to have massively armoured legs.
Dreadnaughts also have armoured legs, but are still weaker than an MBT.
Sentinels really are flimsy and this is represented in their rules.
I'm sure the servitors driving the LRC are well programmed enough not to fire on their own emerging troops. The front doors place the assaulting unit closer to the enemy. And terminators could walk through a hail of bolter fire unscathed anyway.
