^
For the 2nd challenge don't bother killing the harpies just kill the zombie soldiers and archers, oh word of advice save your Rage of The Gods to the last challenge. If you quite you have to repeat the challenges all the way from the start so only play them when you have a good amount of free time.

It doesn't really surprise me, the games were made before Achievements were around (or at least the first one was). Unless the devs decided to put collectibles in for grins and giggles (I don't remember any, but I didn't pay that much attention) there's not much they could add beyond the obvious easy ones in the short amount of time they spent on it (and this isn't saying that's bad, just saying I'm not surprised).

you can do it on a replay file, you know, one in which all your stuff is already at max level.

You can pretty much run through using poseidons rage lvl3.

I think they went this route for a couple of reasons

1- People already finished most of the very hard challenges in the PS2 versions and are not willing to do so again for trophies.

2- Make the platinum easy so you catch a lot of trophy whores and get more sales. I really expect if there is more of these PS2 HD re-releases that most of them will have the same easy achievable platinums.

3- Games didn't start development until July this year so they put in the most obvious and easy trophies so they wouldn't run out of time.