I would say it depends on both your previous experience with dungeon crawlers and your level of Japanese reading ability. Demon Gaze is an easier and more modern game, and it's available in English. Operation Abyss is based on a PS2 game, and so it has older and more complicated mechanics, but also has a higher amount of customization in terms of equipment and stat allocation. A lot of the Japanese for important things like spells and skills is katakana, so even just knowing that is very helpful for not getting lost in menus. There's a
wiki for other things.
I personally enjoyed Operation Abyss quite a bit more than Demon Gaze. Although, that's probably because I played Demon Gaze first and had a general idea of how an Experience DRPG plays.
And as for the 3000 battles trophy, yeah, it's a bit out of place compared to the other ones. Every other one can either be obtained by playing through the game and post-game normally, doing a few side things, or by doing a little bit of grinding.