Lex may seem extremely weak against Tyra, but when you have no mobility against a flanker there are plenty of situations where you could be caught off guard. When countering Tyra, Combat Slide (and the Heroism legendary) is the way to go.
Contrary to what you may see here, setting your rune in the enemy's line of sight can work to your advantage if you have the Inner Strength legendary. The enemy expects you to be weak and on cooldowns when you return to your rune.