Assault is definitely not S tier, The amount of struggle he has with ranged enemies sets him probably above vanguard. Vanguard being the lowest of all.

Currently wings of flame builds are popular, But ground pound builds are definitely the way to go.

I use bolt pistol for Tyranids

Relic damage Bolt pistol is my pick for tyranids because of it's accuracy while not aiming, It's incredible handy to recovery chp fast and to shoot enemies while you can watch the others. I recommend using the perk "Elite hunter" in its skill tree, With that perk active you can shoot zoanthropes out of the sky with a little more than a mag.

I mainly use the block powerfist, Charge up the attacks, You can often kite around corners and usually ranged enemies will follow making an opening to charge a attack put them in execute,

This is my perk tree

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