I dont know about part 3 since i am waiting for at least a mtl translation, but from my experience in part 1 and 2, Luka is a horrible dps actually (unless you are using gravity pleasure but thats brokean as fuck).
really? he and king of grangold carried my ass back in the days, in part 3 he Is pretty weak but thats becuse the celling spike is just too high, must be a different in our playstyles but i do think he's one of the best dps of part 1-2 (until collab)
Generally, humans are weak compared to the other Monsters since they are relatively limited in terms of Race grinding (even considering Promestein's race transformations). The Main-game meta (until Collab Event) is to do whatever is possible to beat Superbosses (preferably in order of strength) to then recruit them and beat each of them while slowly gathering strength in this way.
The real difference comes in Grinding for all Jobs&Races, specifically the Race part, as the Meta party members usually have a lot of innate races available to them (even more with Promestein race transfers), aside from some innate good Traits.
Luka, until Collab Event, was basically just along for the ride by comparison. While he has some story-given skills (such as the Spirits), he quickly gets powercrept by even the earliest Superboss Nanami. However, with the Collab Event, the access to Nightmare damage on all his physical attacks does give him a more sizeable advantage, which is then used in the broken Nightmare Gravity build (& Death Claw from Sorcery skills). Fyi that broken build is one of the bread&butters for doing late-LoC floors (when enemy HPs spiral into virtual infinity), that alongside Barrier-applying Damage-nullification Full-Evasion builds.
The Elf Princess is broken AF. Her unique trait allows her to equip EVERYTHING. Absolutely worth the trouble of recruiting her.
Would have to test her mettle by full-grinding her stuff (if my past Colliseum Grind Macros still hold up to Part 3 or if there are better non-LoC grind methods)
By the way, did you MTL via Mtool? I tried loading the game in Mtool, but it keeps crashing after 1h+ of loading due to runtime error in the application (most likely running out of game resource in the meanwhile).