Lex deals too much damage to fight on your own. If you're under attack from him, use Shadow Travel and move to a safer location, then regroup with your team.
If you suspect Mal'Damba has his ult ready, try to space yourself a bit from the rest of your team. Dread Serpent will knock you out of the air if you're flying, and its hitbox persists for a few seconds after it triggers.