Hajime is both fast and strong enough to catch and deflect the thing in mid-air. It also isn't curled-up the majority of the time, when he breaks the first horn it is just standing there in front of him. You really think he couldn't just shove his war pick in the eye instead of breaking a single horn off?
He takes a step closer and it jumps again, lmao? Not to mention, even if you do hit it in the eye there's no guarantee it doesn't just wrench away the warpick (jaw, tail, headplate, take your pick) and now you're disarmed. Congrats, the wreckingball will now pound you like a meat mallet.
It's not literally impossible for him to gamble on a lucky crit, but at that point even a single mistake means you're going to trigger bloody battle before the worm even shows up. Whereas if he keeps his warpick in hand and goes for easy deflects, at most a mistake costs stamina/fatigue and a bit of blunt chip damage.
(not to mention, it is curled up a majority of the time, the manga just timeskips through the fight because it's not fundamentally an action manga that spends dozens of pages on these things? And i have no clue where you got "he can catch it midair." He can deflect it midair because that's just whacking a giant pinata, you swing and most likely you'll knock off a spike. The worm diving into the ground probably takes entire seconds because the damn thing is
long. )