JackAxe, watch Lorax86's vids.
http://video.google.com/videosearch?q=re4+lorax&so=0
He doesn't have the highest scores but watching his videos helped me go from struggling to get 10k to easily getting 60-70k every time with any character on any one of the 4 stages.
Just do what he does.
Best advice I can give on Mercs...get all the time bonuses first, then grab a combo bonus (to start earning a high score on the 1st kill rather than the 10th), and then go crazy.
-Grenades work very very well.
-Leon sucks.
I rate Mercs characters as follows:
tie 1st - Wesker/Krauser. Wesker - thrust punch after a flash grenade, rely on weapons when necessary. dump the silencer on his pistol for the case space if you need it; pistol is a great headshot/1hit kill. I relied on his rifle a lot (even in somewhat crowded spots) Magnum for crowd control.
Krauser - start out with flash grenades then the double kick, use the bow only when all your grenades are gone or if you have the distance or need to keep a combo going.
3rd - Ada (even though her lifebar is weaker than $%@! and she takes damage like a mofo) very nice weapon selection. rely on the rifle 1st then the TMP. pistol as a last ditch.
4th - Hunk - grenade grenade grenade! tons of grenades are given to him. use 'em a lot. when you run out, TMP the enemies to death. (TMP is the only other weapon he has so there's always plenty of ammo.) Neck snap if you're daring, again though I don't bother with it. Neat attack, leaves you pretty open though.
5th - Leon - rely on the shotgun and grenades. pistol last ditch.
Once you unlock Wesker and Krauser you'll get really good at high scores and combos (especially with Krauser). His plaga claw/arm move is great for getting out of a pinch, killing the bosses, or as a last minute effort to keep a combo going.
One other huge thing worth mentioning...(well, two):
On the castle level, watch where Lorax goes and where he ends up. If you stay off the ground + 2nd floors of the castle, you will never see a Garrador (the boss on the castle level) - not once. (Assuming you don't open the doors at the very top of the castle which goes into a room with 2 Garradors.
) Normally the Garradors (after awhile), one will come running around a corner in a beeline straight for you, if you're on the ground floor (bottom of the stairs near the cannon where you start) or the 2nd floor. (On the 3rd floor you will never see one.)
On the waterworks stage - kill both Dr. Salvador X's first. (before you grab the time bonuses) It will give you a lot of breathing room because you won't be worried about getting your head chopped off by a free swinging chainsaw lunatic (or his brother).
Ok...one more.
Village - grab all the bonuses then go to the 2nd story of the barn and wait. knife or shoot thrown weapons, keep kicking the ladder down. When the room is nice and full of enemies (after about 30-40 seconds), grab the time bonus, shoot the barrel, jump down and run out the door and prepare for madness (enemies will start spawning everywhere). Your ears are your first warning that Bella Sisters are coming; watch out for them near windows because they will stick the chainsaw through a window and you can run into it and get your head chopped off. (It's happened to me more than once
)
Again - watch Lorax's vids to see where he goes and what he does, it will help you a lot.