Arup (5/21) and his pace partner Krisha Das (4/27) - both hail from Barpeta - maintained a tidy line and length on a pitch with little assistance for seamers.
Saurashtra put up yet another atrocious batting display to be bundled out for 81 in 39.5 overs in their second innings, their lowest against Assam.
Chasing a paltry 64, Assam lost opener Rahul Hazarika (8) for team's score of 11 but Rishav Das (30 not out) and Sibsankar Roy (24 not out) chased down the target in one hour and 19 minutes after Saurashtra lost wickets in a heap in an extended first session.
Rain had robbed four sessions after a washout on day one, and a delayed start the next day, but heavyweights Saurashtra's meek surrender -- losing five wickets for nine runs in a space of 35 balls in the post-lunch session -- meant it hardly mattered to Assam who won with a session to spare on the final day.
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Their second loss from four matches pushed Saurashtra (six points) to bottom of group B while Assam jumped to sixth place with seven from five matches.
Krishna gave the first breakthrough of the day when Sagar Jogiyani (0) edged an outswinger at second slip before Arup joined in dismissing Dharmendrasinh Jadeja (12).
Amitoze Singh offered some resistance and counter attacked against Abu Nechim with three boundaries as the seamer conceded 13 runs in his first over.
But the experienced Nechim fought back to castle Amitoze who missed the line to be out for 30, something that triggered the collapse.
Saurashtra were 39/6 and then 69/8 as their late order batting had revived their first innings total to 153 after Assam opted to field following the first day washout.
"I don't want to make it an excuse but toss definitely played a role," Saurashtra coach Sitanshu Kotak said.
"Having said that, it was definitely not an 80-run wicket. Our batting is letting us down. We have not batted this badly in the last two seasons."
Brief Scores:
Saurashtra 153 and 81; 39.5 overs (Arup Das 5/21, Krishna Das 4/27).
Assam won by nine wickets.
Points: Assam 6, Saurashtra 0.