-When fighting the Stygian, you can max out your Chaoseater Blade by attacking it repeatedly. Begin the battle with the Stygian by killing phantom guard soldiers while riding Ruin. After the Stygian frees itself, ride along side the Stygian and strike its body. Do not strike the helmet or it will break off and the Stygian will attack. Equip Combat Lore to increase the experience you get from each strike; Bloodthirt will replenish your health. The Stygian will sink under the sand and resurface. Repeat the process to get the maxed out Chaoseater.
-If you mess up and knock off the helmet, you can exit the battle. After removing the Stygian's helmet, ride Ruin to the closed doorway during the battle. You will jump off Ruin and have control of Ruin. Keep your back against the door and wait for the Stygian to come right at you. He will knock you right through the door and you can go back to Vulgrim. Come back to fight the phantom guard soldiers and the Stygian when you are ready.
In The Black Throne, while heading to the Third Guardian, you will come across a bridge with spinning portals (The Black Throne, The Third Guardian III). When you reach the other side, you first be ambushed by a Rot Mauler and a Shield Lord. Defeat them and your next opponents will be one Shield Lord and four low-level soldiers. Do not kill the Shield Lord; instead, keep killing off the four soldiers and they will keep coming back, giving you as many souls and as much experience to max out your weapons as you want. To make things easier, run away from the Shield Lord whenever he gets close, and the four soldiers will follow (they are quicker than the Shield Lord so you should be able to get some hits in, run away when the Shield Lord gets close, hit the soldiers some more, rinse and repeat.
Begin the "Gory End" Dark Realm challenge in the Choking Grounds, which nvolves fighting a big group of bats that can be killed in one hit by using your sword attack. War will kill a bat while it is still in the air, and you can jump off the target to another bat without hitting the ground. Jump from bat to bat with one hit kills until the achievement is yours.
To find the final Wrath Shard, use the Serpent Hole to reach The Ashlands. Take a right out of the small cave you are transported to, and continue down the path until you reach the area with the three monsters hanging from the ceiling with the wire that you can jump to in the middle. Jump on the wire to the left of the monsters, and continue left. Grab the growth, and pull yourself up into the opening. Then, look to the right and up. You should see an orange object you can use to grab with the chain. After using the chain, walk slightly further into the area to find a chest with the last Wrath Shard.
Begin the battle with the Stygian (Ashlands Boss) by killing phantom guard soldiers while riding Ruin. Equip Combat Lore to increase the experience received from each strike and Bloodthirt to restore your health. Once the Stygian frees itself, ride alongside it, and attack its body, but do not hit the helmet or the Stygian will attack you. The Stygian will disappear under the sand and resurface. Repeat the process to increase your Chaoseater Blade level. If the Stygian attacks, ride Ruin to the closed doorway during the battle to jump off and have control of Ruin. Keep your back against the door, and wait for the Stygian to charge at you. You will get knocked through the door and can return to Vulgrim to reset the battle.
Bombs growths on the wall in the game can be activated by throwing anything at them, such as a pole or a car. The Crossblade isn't necessary, and this method gets your some items much earlier in the game (like the Hoardseeker in the Twilight Cathedral).
You must throw a car at a helicopter in level 1. Stay in regular form and do not transform into a demon to be able to throw a vehicle. When you see the huge rock monster in this level, look for the helicopter. It is the only place it will appear. Hit the helicopter with the car and the achievement is yours.
In the Black Tower, when you get on the elevators, look up and you will sink into the floor of the elevator as you move up. If you don't jump right away, you will fall through it and never be able to get the elevator to come back down. The only way to fix this is to start a new file.