First off beat all the modes up to this one, there's 4 modes, however everything after hero 30 will take less than 5 minutes a piece. Sadly they are much shorter than the original.
When I first beat this for PSP this was quite hard, in the XBLA version it's almost as hard if not harder (You be the judge)
There's a few big tricks.
The very base one is never die. This is something that can be useful in a few levels to get back to town faster, it can only be useful at one place in the 300.
Second, keep freaking moving. You have 60 seconds to get out of the first area? use 20. Time will get tight at the end so more time there the better, in addition that gives you plenty of time to gain power.
I'll give you basic tips to beat the level, not to get everything. There's certainly far more content then there needs to be for this level.
To start, get through the first area, kill anything that attacks you but don't power level. It really shouldn't take you much more than 30 seconds to get to the second area.
In the second area, start picking up some weapons, blowing all your money here isn't a big problem as the weapons are decently powerful, You'll have to travel to the far east side of the level to get the princess, NOTE: If you're low on life here, die, this can save time walking back (however if you can there's a town south of the princess with a 400g weapon that's quite nice). I have gotten past the third area at 60 seconds used, very good time in my opinion.
Now going to the third area you should try to get a herb in the second area or third. don't be afraid to use it but they are very useful, make sure to save one after the fourth area.
In the third area there's what looks like side bosses, east and west of the castle down a ladder, ignore this or don't. I ignored them. However there's a cave that has some good items for sale.
Up to this point it should be pretty easy, the fourth area is where things start getting hard. However it also starts selling the brave armaments. These are some of the best items in the game, buy as many as you can.
In the fourth area there's two bosses, they are both quite powerful, but leveling up a little, you can beat them with a herb and that gives you the most amount of time. Once you've beaten both try to buy all the brave items you can (including the item in the mountain area between the third area and fourth area.
Note, there's a 5th area after the fourth. Once you've beaten the two bosses the center teleports you to the final area, if you have 100 seconds you'll be pretty golden. you'll want to fight the final boss as soon as you can. Strong enemies do appear in the south with no enemies in the north, use them wisely. The first final boss fight is pretty easy and shouldn't need much explanation.
Once you beat her you have to run through the final area again. At this time you'll probably be very short on time, use it to the best of your ability, level quickly, then go back into the final fight. You need to save a decent amount of time (and a herb if you can) for this fight. In the final fight, there's a mirror, when you see the boss move forward and back, rush the mirror, destroying that mirror means you've won.
So that's what you need to do in five minutes. There is no statues so it's a straight 5 minutes + time in towns, it'll take 2-3 attempts for new players to even see the boss, but the big factors are avoiding wasting any time in the first 2-3 areas, and leveling a little more in the 4th and 5th and the final boss shouldn't take more than a few attempts.
However once you beat this you'll be in time for the hardest mode....