This is a quick guide on how to make the most out of your autobolt melee character via CD in Mouth, etc. I'll try to shorten the explanations as much as I can; for comments and questions, just post below.
The following equipment build is for Ragnarok Online MSP. Some items may not be available on other servers.
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Suggested Equipment Build for Melee Autobolters
Transcendent Swordsman, Merchant and Thief
Limitations and Precautions:
Must be at least level 70
Must have decent INT stat for MATK, and ASPD for increased proc chance
Must be wary of monsters with LEVELS 2 to 4 element properties (especially Fire, Water and Holy)
Must have an element endow to hit Ghosts
Core Items Required:
CD in Mouth (OSM) - casts Fire Bolt, Cold Bolt, etc. for a few seconds by chance. Full description here.
Shang Tsung's Rainbow (OSM) - needs a weapon with certain cards to utilize its effect. Full description here. iRO version(?) here.
Elemental Sword  - has a chance of casting one full bundle of bolts per hit under normal circumstances. Full description here.
Laurell Weinder Card
Red Ferus Card
(optional) Hill Wind Card - can be replaced by other weapon cards depending on class and/or personal preference
How the build works:
The Elemental Sword is the real keystone of this equipment build. Whenever the character attacks by physically hitting the monster, and whenever one bolt triggers, the Elemental Sword will trigger a chain of bolts in quick succession. Alone, the sword has a dismal 5% chance of triggering its own effect, but the rest of the items especially CDiM will ensure a significantly higher chance of it occurring.
Whether manually casting a bolt (via Intimidate, Fireblend / Ice Falchion swords, or scrolls) or via autocasts doesn't matter, Elemental Sword will trigger its effect regardless of how its wielder invoked the bolt spell.
Here's a couple of quirks on the Elemental Sword.
It will cast the following in precise order: [Cold Bolt -> Fire Bolt -> Lightning Bolt -> Earth Spike]. If for some circumstances a bolt was cast other than Cold Bolt, the sword will skip the other previous bolts on the list. For example, Lightning Bolt was cast first: Elemental Sword will cast Earth Spike immediately after, skipping the Cold Bolt and Firebolt altogether. This feature can be actually useful: for example, if the monster is a high-level Water element, Cold Bolt can be skipped by casting Fire Bolt and the sword's other autobolts will follow shortly after.
If a bolt misses on a monster due to its element property, the sword will not cast the rest of its other bolts. For example, On level 2-4 Fire monsters, Elemental Sword may autocast its Cold Bolt, the Fire Bolt will miss, and Lightning Bolt and Earth Spike won't be cast. The sword's effect will totally miss with level 2-4 Water monsters. Wind and Earth monsters will resist their respective elements, but not before Fire Bolt / Cold Bolt is cast first. Other items like CD in Mouth can partially address this issue.
Worried about the Elemental Sword? Don't worry too much, because there are still the other items as backup.
CD in Mouth is going to be your real main source of preliminary autobolts along with Shang Tsung's Rainbow to empower your Elemental Sword with. It also grants an ASPD increase when paired with Shang Tsung's Rainbow. There really isn't that much any other explanation necessary on this item being included on this build.
Shang Tsung's Rainbow is a headgear that doesn't really have many effects on its own, but must have certain cards compounded to an equipped weapon to enable its own autocasts. This is where the Elemental Sword comes in - in addition to its own effect, it also has three card slots. Of the five weapon cards that can be tagged to the headgear, I recommend the Laurell Weinder Card first and foremost, followed by the Red Ferus Card. Remember the sword's [Cold Bolt -> Fire Bolt -> Lightning Bolt -> Earth Spike] cast order? Hill Wind and especially Pitman cards won't help in the headgear trying to trigger the sword's Fire and Cold Bolts, and the Death Word card doesn't synergize with the sword.
Once you've collected the required items and compounded the cards into the weapon, using them is relatively simple. Just click on a monster (make sure /noctrl is set to ON), let your character whack it a few times, and watch with glee as the monster gets riddled by a rain of bolt spells.
Much, much more bolts than what you'd normally get with just a CDiM.
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Disclaimer: Probably somebody else has already figured out the build above before I did, but AFAIK nobody has yet shared this in an online forum. Furthermore, I shall not be held responsible for any unforeseeable bugs and/or nerfs in the future.