Having spent well over twenty hours (and counting) traversing Raid Mode, I wanted to take a quick couple of minutes to pass on some tips to newcomers who might just now be trying it out in it's console/PC iteration.
What is it? Raid Mode is a single player or two player co-op mode that pits you against the monsters of RE:R in a (usually) linear map where the goal is to make it to the end quickly. Completion time, enemies defeated, accuracy, and damage taken are the main factors in getting an S rank but there are also badges for defeating all of the enemies, taking zero damage, and attacking a level at an equal or lesser level than suggested. Getting all three badges at the same time gives you an additional badge, Trinity Bonus. Badges and higher ranks give more points which can be spent in the Store upgrading your weapons. (Your first Trinity Bonus also unlocks a new character.)
Tips for low level players: If you don't want to take advantage of the two cheap DLC packs Capcom has released with a number of low level weapons and parts, my suggestion would be to log in and synchronize your account at ResidentEvil.Net. The site will provide you with RE Points which you can use to send items and weapons from the website to your game. The more you play, the more points you get. The first several levels can be a bit of a slog, so being able to send yourself a few more powerful weapons will make you more efficient in the early goings. Additionally, the @RE_games Twitter feed will occasionally offer Present Codes you can enter into the site for additional weapons.
Unless you've fully beaten the campaign mode on every difficulty level and gotten every SP achievement possible, you likely won't have many characters to choose from at the outset. As such, you should focus on using Jill and Parker to their best advantage.
Jill is a handgun/machine gun specialist, so finding high crit parts for the pistol and longer magazines for the machine gun should be a priority.
Parker is a shotgun specialist, so fire rate, reload rate and, most importantly, stopping power should be where you spend your money. Crowd control becomes a priority very early on. Having a nice shotty with a lot of stopping power means enemies will spend most of their time on their ass where you can easily melee them to death.
Speaking of crowd control, pulse grenades will be your best friend. Upgrade the amount you can carry first, as soon as the game allows. Grenades are a useful tool on Chasm difficulty but they do not level up as you do and, most of the way through Trench difficulty, I've yet to get an option to buy more powerful iterations. Even enemies with a weakness towards certain grenades will take dramatically less damage on Trench and Abyss difficulties. However, pulse grenades will always be able to daze foes. This is the ultimate crowd control device. It'll get you out of trouble when you're surrounded and/or open up the oozes so ridiculous damage from a fully charged melee attack.
I decided to tough it out and play Raid Mode solo for the first half of Chasm and, in retrospect, I really wish I hadn't. Solo is good for hunting badges but co-op is really where it's at if you want to S rank a stage. The game doesn't seem to scale in difficulty to having a partner so an extra body means more damage dealing and quicker completion.
My experiences playing with strangers has been almost entirely positive. The only downside is that no one appears to use voice chat, which leads to frustrating tactical situations where you'll be plugging bullets into an ooze that has crept up behind your partner and it wallops them for huge damage because you couldn't warn them. Co-op play is great but co-op play with a friend over a headset is ideal. Also, I have a feeling that participation will drop off pretty dramatically for this game in the next few months since Capcom haven't announced more stages via DLC, just extra characters.
I've played over 23 hours of Raid Mode so far and I'm only at level 25 and most of the way through the second of three difficulty levels. If you want to max out your characters for achievement hunting purposes, you're looking at probably around a fifty hour investment. The console versions of the game even have an extra long bonus map, The Ghost Ship, which has a recommended level of 50, so there's plenty of gameplay to be found in Raid Mode if you're looking to extend your RE:R experience beyond the admittedly goofy campaign. Going in, I didn't think it'd be worth my time but it's quickly become the highlight of the game. If you plan on renting it or picking it up cheap when the price drops, don't ignore it, it's actually well worth your time.