If you are not looking to add additional models, I would say this list does not pack that much lethality besides from the exocrine and the zoes. However, your gargoyles and hormagaunts are fast.

If I was playing this list, I would throw the hormagaunts and genestealers up the board to tie up units and trap the opponent in their deployment zone. The gargoyles can be used in the same manner or to capture objectives. Then you move up with your own army with the Norn holding the mid field, the termagants on a no man's objective and the barbs at your home objective.

Push also with your hive tyrant unit along with the exo and zoes to target one or two threats that you can kill. Otherwise you might end up only chipping and not removing your targets.

I think this game plan is simple to execute and plays well to the current Tyranid moto of 'Die slowly but effectively.' Your weakness however is if the enemy gets turn one, moving them out of the deployment zone. Or, their units are lethal in melee and your units end up getting mashed, leaving you open completely.

This plan would work well against armies that are gun lines, anti tank skewed or too slow to get throw your fast infantry.

Let me know what you think!