The difficulty achievements do not stack. Therefore, you should play the other 2 difficulties first to get a feel for everything. On all of the difficulties, play at a speed that allows you to complete that campaign. Levels are longer on the Serious campaign, enemies have more health, and all enemies fire A LOT more projectiles at you; trapping you in a corner at times. The shield is easily the strongest weapon in the entire game. Tap your shield and the shield will clear bullets and damage/kill enemies around you. DO NOT be tempted to grab a “Ghost” power up or 1UPS. Let them come to you, or clear everything out before you grab those. Don’t take any unnecessary risks, and you should get through with little difficulty.