I can't see why you would go toe-to-toe with tyranids in melee. Maugan will help you a lot.
A lone avatar in melee (basically unsupported, becuse you have nothing that can really help him in melee) can be tarpitted by 30 gants or exploded by a couple of zoantropes... also, you will be able to reach melee in turn 2 at the very best. Maugan can be protected by other shooters around him to give ablative wounds and will start reaping from turn 1.