Devil May Cry

On level three just after killing the Phantom, collect the Pride of the Lion, but do not exit the level. Instead go back through the door to the broken bridge and fall into the water. Thus you will find the first of the 12 secret missions.

Hidden Combos:

There are little moves that you are not told about in the manual. For instance, if you get the stinger attack for Alastor, you can also do an amazingly long jump. Simply go up to a ledge like the broken bridge level and perform a stinger move, instead of stabbing thin air he'll do a long jump. Not helpful there since you'll end up in the water but it is useful for getting the blue orb on the platform in mid air above the door there.
Another one is combos. If you tap X three times he'll do a normal combo, but if you vary the timing of each tap, say tap X twice then wait a little then tap X again, he'll either swipe twice then do a cool combo, or a cool little stabbing move. The more times you time the X while he's stabbing the more stabs he'll do.

Good Combo:

Press O twice then wait for 1-2 seconds then press it again. Dante should then do a combo of about 10 hits.
Hints
You can save, reset and fight bosses over and over again as the save only save your stats and items. This is an easy way at the beginning of the game to rack up on red orbs and upgrade early. You will want to do this because the bad camera angles will cause you to get hit where you probably could have dodge had you seen what the enemy was doing. This is especially easy with fighting the phantom spider the second time in the hallway. He gives you 500 orbs every time you kill him and he is very easy to kill. If you have not figured it out you can do several things to kill him. You can run and wall jump all the way back to the wall thus avoiding his fireballs or you can use your demon transformation to air attack him with your thunder. Either way, you simply kill him, save and reset until you get everything that you want.

When taking the Lion's path across the bridge. It will break and you will fall in the water then fight underwater and teleport back up to the other side. Go back to the other side either jumping onto the broken fragments of underwater and there will be a blue orb there.
At the door on the outside of the room with the Lion's symbol there is a floating piece of bridge fragment over it with a blue orb. To get to it, you must have Double Air Jump upgrade.
After fighting the phantom spider the 1st time, save your game and at the beginning of the next level, you will start at the Judge of death door where you met the phantom spider the 1st time. Re-enter that room and you will be give a secret challenge to kill 100 baby spiders within the allotted time. If you succeed, you will get another blue orb.

After Killing the phantom spider the second time in the hallway and after succeeding in the 1st secret level spider baby challenge, go back to the room near the picture with the strange aura. There will be another secret level challenge there where you have to kill 100 baby phantom spiders without any weapons. Simply run around and stomp on all of them.

Dante Must Die Mode:

Defeat Legendary Dark Night Mode to unlock Dante Must Die Mode.

Demon form:

Defeat Dante Must Die Mode to unlock infinite Demon form.

All Star Group Photo:

To unlock All Star Group Photo you must get S Rank on all missions.

Easy Automatic Mode:

Clear any mission from 1-3 using continues, and there will be an option at the start of the next mission asking if you wish to change to Easy.
Hints:
In level 3 and beyond when you fight the Phantom boss, the best trick to bet him is to use the Alastor attack Air Vortex(Aim for his head or the top of his back, this works best when you confront him in the hallways), otherwise use a jump attack with Alastor.

Fighting the Death versions of the Sins can be tough; a good tactic is to use the Alastor's Air Hike, the farther you fall the more damage you do. Just hold down R1 to stay targeted on them. This will do a lot more damage than any gun will do if you time it right.

When fighting the last boss, a good tactic in normal mode is to buy 10 devil stars and a vitality star. In the first part of the battle use one then use devil trigger, watch the wicked dragon, repeat. Once you are on the ground, use a devil star, trigger, and jump right in front of him, the lava will only do minor damage in devil trigger mode. Just constantly swing at him, use another devil star once your meter drops to 1 rune. If you get low on health use a vitality star. I doubt this works in Hard mode or higher, but it works wonders in Normal.

Finding the Shotgun:
To get the shotgun for Dante, go into the room next to the Judge Of Death. Go upstairs and destroy the table directly across from the Grim Reaper with the rusty key.

Secret Missions:
Mission 3 - Critical Hit
Near the broken bridge, drop into the water . Point value: 200
Mission 4 - Phantom Baby
Go back into the church where you battled the Phantom. Point value: 100
Mission 4 - Phantom Baby 2
First defeat the Phantom Baby in the secret mission before, then go to the God of Time room. Point value: 100
Mission 4 - The Three Beasts - Located inside room that is under the biplane room. Point value: 100
Mission 5 - One-Eyed Evil - Go back to the room where you first saw Beelzebub. Point value: 200
Mission 11 - Wandering Ghost - Go back into the final room of Mission 10.
Mission 13 - Water Cell - Go to the captain's skeleton in his room. Point value: 100
Mission 15 - Treasure of the Reaper - In the where room you first got the Shield Emblem, look at the skull . Point value: 200
Mission 16 - Stairway of Tranquillity - First beat Griffon, then go back to the coliseum. Point value: 100
Mission 16 - Shadow of Darkness -First beat Nightmare, then return to the biplane room again. Point value: 200
Mission 17 - Blue Gem in the Sky - Cross the broken bridge using Air Raid. Point value: 200
Mission 21 - Hidden Bangle
In front of the God of Time statue, look at the wall. Point value: 200

Cursed puppets:

When facing a ton of cursed puppets, get your guns out...ready aim...fire until they are dead, they slow down when fired at, if they get to close slash them an knock them away then continue to pump them with lead until they blow up into pieces of rag doll! Also when you get alastor you will be begging to get at them! You will run them through....jab them in their stomachs about 3-4 times...throw them up in the air and slice them up...what more can you want in a lightning sword?