As it stands, Lennox is a formidable foe for Goliath and Behemoth, but she suffers against Wraith, Kraken, and Gorgon. The reason for this being that she is the only Assault whose main weapon is more or less useless against a target who doesn’t have to stay close to the hunters for extended periods of time.
The exception to this rule is Parnell. The difference, however, is that he has a rocket launcher which explodes within a certain area around the target. Lennox has to rely on her secondary almost exclusively when fighting the other three monsters, which is jerky and difficult to control effectively.
So, I think a simple way to make Lennox more viable against more than Goliath/Behemoth would be to convert her anti-aircraft cannon damage from normal bullets into explosive damage, akin to Sunny’s mininuke launcher or Hyde’s gas grenades. To balance the more reliable damage output, I would suggest lowering it by a small amount.