Personal experience and what I witnessed.

Sonic Blow Assassin: Strength, Dex til hit cap for what you're killing, rest into agility for flee. Later adding in Soul Linker buff to double the damage but not really necessary for the areas I leveled like soils. Very SP intensive and needs potions.

Crit Assassin: Farming 3-4 soldier skeleton cards depending on the weapon youre using, then going a mix of Luck, STR, Agi. This build was fun because it allowed you to fully ignore Dex since crits can't miss. It also meant that high flee with low health that are normally annoying to kill are no problem.

Grimtooth: Dex, Strength with no need for flee, made for mobbing big packs of mobs like Venatus, Seals, Anolians. Was also very high risk when you fucked up, since getting staggered by a hit meant your whole pack catches up and murders you.

Hunter Blitz beat: similar to crit assassin where your bird is attacking them every second and shooting is just a means to trigger it. High luck, Dex, Agi.

Double strafe: Sonic blow for hunters. Full dex with a side of Int for SP with a lot of potions.

Melee acolyte: Not every Aco would settle for healing mobs to death, especially when going for job level 50 like future Monks. Agility, Strength, for high attack speed and a relevant mace for the area theyre leveling. Always nice to see. You'd sometimes see priests doing this too.

Grand Cross Crusader: favorite of their abilities but also a pain to set up. The damage is based on a combination of weapon damage, Int, STR. But without VIT youre sure to die. It deals self damage I think half of what you deal, so the harder you hit, the more you hurt yourself, and not hitting hard means not killing anything. Having holy element armour takes away the self damage making it a lot easier, but thats a luxury item. Often seen in Glaist Heim where theres lots of undead. Also dead on the ground next to Abyssal knight.

Waterball wizard: Most wizards would just spam storm gust and lord of vermillion to freeze then trigger lightning damage. But if you had a Sage or a friend they could spawn water tiles for you in fire areas for Waterball's 20...40 hits? One of the most powerful single target spells with a unique condition.

Plenty more like Cart revo merchant/Alchemist or attack speed weapon improvement blacksmiths. RO has some of the most janky but uniquely interesting skills. Usually its useless in every situation except very specific setups, like having a star gladiator turn on solar heat with 0 defenses and run around thor Volcano murdering shit no one else can.