Dread banner is the easiest way
40 point banner to cause fear is awful. It also requires you to put a BSB in the unit AND not give it the standard of HG.
death magic also gives you this option
You shouldn't rely on magic for a unit to survive.
if you loose a combat with with elves who have greatweapons you are doing something wrong
Easy to say. But games don't always go to plan. The best general prepares for times when your unit doesn't manage to achieve optimal positioning.
Yes, it's always nice to envisage playing a game where your executioners get a rear charge on the enemy while your chariot charges the front and your cold one knights go in via the flank, but to assume that this (or other less exaggerated sequences) will always happen is not a good idea.
For example, lets say your unit of execs is in combat with a unit of chaos knights. You win the combat. He runs away and you charge him down (as you have to, due to hatred) but fail to catch him. However, your flank is now in charge range of his chaos trolls. Next turn he charges you, survives your attacks (S6 vs T4, followed by an armour save and regen saves), then slaughters you en masse. You're stubborn but, oh dear, he causes fear. So off you go.