Ward (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays .)
Spider-Rex is a mono-green beater: suit it up with auras and equipment, then swing for lethal commander damage through its built-in trample and ward protection. Ramp hard early, drop Spider-Rex, and use the rest of your deck to make him bigger or harder to block while reach lets him also hold down the skies on defense.
Ward makes him resilient to spot removal and a sticky Voltron threat
Trample is built in, so pumping his power converts directly to damage through chump blockers
Reach lets him block flyers, covering mono-green's traditional weakness
Mono-green has the best ramp and biggest pump effects in the format
No evasion beyond trample, so wide gangs of blockers and fogs slow him down
Mono-green struggles with interaction, card draw quality, and answering noncreature permanents
Six mana means he often arrives turn five or later, behind faster decks
Board wipes and edict/sacrifice effects bypass Ward entirely
Single-creature game plan is vulnerable to a well-timed exile or bounce that dodges Ward