I'm assuming that the longer it takes you to reach White Forest, the less pokemon there will be (and I think it is a maximum of 5 if you haven't used Entralink to get to Black City).
If you don't speak to the NPCs found there, their hidden value gradually decreases by a set amount. Once this amount hits 0, they leave and you can no longer find the pokemon linked to them, reducing your grass. This is the reason why changing the clock empties the forest, and the only way to fix this is by going to Black City.
Also, where is the IV checker? I've found the Hidden Power guy already, at least.
Dark Pulse is an excellent move on it because it allows you to beat Jellicent, Reuniclus and a few other common pokemon that you couldn't normally defeat.
Hydreigon is also pretty quick compared to the pokemon of this gen, so you can nab the flinch chance. I currently have my in-game Hydreigon with Dark Pulse / Surf / Fire Blast / Draco Meteor and it works perfectly. Being able to take out 2 of the 4 Elites its always nice.
Alternatively, you could run Crunch. It gets a Hustle boost until it evolves, and hits said ghosts and psychics on their weaker defensive stats.
EDIT2: I've now played through the game with all the starters, and I have to say that Samurott worked best for me. Maybe its because it shines compared to the other Unova waters (while Emboar has stiff competition from Darmanitan and other fire-types), but its mix of power and decent bulk helped me out quite a bit. Emboar comes in second with good power, but its low speed and relative frailty were a bit of a let down. Serperior is horrible, period. Such a disappointment following Sceptile and Torterra as far as grass starters go.
Next game, I'm trading in eggs and doing a run-through with some of my favorites. Hoenn trio, Zoroark, Lucario, and Salamence. Should be fun. Hopefully the DW maintenance going on right now will fix the issue I'm having, too.