You can’t/wont kill it outright with a melee setup but melee KI1 can get you down as far as 50-60%. I think my groups clear was 64 or 59.
IIRC, there was a JP group that did V20 KI1 with melee setup. Forget his name, but they used a RDM with Mandau/Mercy Stroke in the comp, WAR tanked Arebati, and TP on Raaz while WSing Arebati and resetting its TP.
You probably could now too, with gear creep/higher average party MLs etc. Years ago when my static was spamming V25 KI1 Arebati (this was early after release), our best run we got it to 40% (and intentionally held it so we wouldn't push to 39% for the 2nd add). This was pre prime, so didn't have additional WS (Ruthless Stroke/Diarmuid) to counter the WS wall
DNC DRG (Empyrean) WAR (Shining One) RDM WHM BST
BST would Killer Instinct (plantoid) prepull > Leave > Bestial Loyalty > Slug > Corrosive Ooze > Leave > Fatso Fargann > Unleash > TP Drainkiss spam. Box Step 10, Dia3, Haste II/Haste Samba. Yes, no songs rolls or Geomancy, just straight DPS. We pulled Arebati down the stairs closer to the middle of the arena for space.
DRG/WAR/DNC would burn any SPs it could at the very start, during Unleash spam (assuming it didn't conflict with KI2). The key was Angon (after Ooze wore)/Warcry/Tomahawk/Climactic window; we would specifically announce when these were ready and do most of our burst damage here. Normally would bring Arebati to around 79%-83% within a 90 seconds or so. DRG would Super Jump somewhere before 75%, so that DNC was intentionally #2 on Arebati's Hate list (WAR will always deal most damage, or at least did back then with Mighty Strikes WS spam). At 74%, DNC would Fan Dance/Chocobo Jig and run up the stairs, as Raaz would beeline directly to DNC, and RDM would follow to apply Bind/Gravity/Slow/Para. Geriond was the RDM, and iirc, he would intentionally Immunobreak the Raaz but never land sleep on it, so that the next Sleep after the subsequent aura would make it land faster. This was during Yaegesumi, so the DPS was halted for 45s anyways.
After aura wore, rinse repeat. We would see 1-2 more auras during the fight, which would break the Raaz's debuffs off, but he had a long run to get back to us down the stairs. We'd quickly proc Arebati with crit WS, then RDM would regain control of the Raaz. The BST (and sometimes RDM) would join with Tauret and Evisceration only to remove aura (Gleti's gear idling gave him enough TP). However, now with primes, even a RDM
could melee and contribute damage. Can't remember if ours did with Mercy Stroke.
We got so good at doing this, we eventually had like 3 minutes left with Arebati @45%, so we intentionally died so weakness would last less time for the lobby job change for KI2. I'm sure you could drop maybe the BST for PLD or something and straight tank it, but we were doing 2KI V25 so we needed PLD for second fight. Or bring a buffer and just re-apply regularly. We had to deal with full dispel occasionally because we didn't have someone who could play a good SMN at the time as a true TP reset strat, because he was also our Empyrean DRG.
We made sure to only use our buffs/debuffs in sync when we were ready to push damage, then just banked on RNG not hitting Glassy Nova. We got unlucky occasionally, but at least without a BRD or COR there were very little buffs to lose (Haste, Haste Samba, Berserk/Warcry, Killer Instinct), so the dps loss was not as significant. BST was applying TP Drainkiss every 30 seconds. Didn't prevent every TP move, but every little bit helped. I can't recall if we did Subtle Blow+Auspice sets at some point just to reduce a little bit of the TP feed. We may have.
On V20 you only have one add to deal with, so as long as you can control it and quickly remove the aura, you might be able to power through it and just straight kill it melee style. Arebati's non-threatening from an offensive perspective, so you never really need to worry about dying besides from the Raaz. Encumbrance, Amnesia, Dispel, all annoying debuffs to contend with.