Gruul Ponza ramps into big threats like Annoyed Altisaur and Avenging Hunter while using land destruction (Mwonvuli Acid-Moss, Thermokarst) to slow you down, so Nihil Spellbomb offers card advantage and can disrupt Malevolent Rumble/graveyard value while helping dig for answers. Arms of Hadar is only a marginal upgrade here since most of their real threats are large (Boarding Party 6/3, Annoyed Altisaur 6/5) and survive -2/-2, but it still cleanly answers Arbor Elf and Eldrazi Spawn tokens that ramp them. Suffocating Fumes and Echoing Decay are redundant or too narrow against their big-creature plan, so trim one of each in favor of the more flexible cantrip. Lean on Counterspell and Snuff Out to answer their haymakers before they take over the game.