There a little trick that people forget or just don't think about: "retreat".
When your team is losing, just "retreat", you'll get back your char even the dead one.
Rethink yur strategy and then retry. That way you'll have nearly all or all yur chars full life ready for the big battle.
Even if it's a strong opponent, you'll have 3 times more firepower than him.
So yes, it's difficult to fail.