Unlock and complete all stages in the game at least once to unlock Luigi. Then, hold L at the matchmaking screen to play as Luigi. If you do not want the L button messing with your item selection, you can also press A to choose the stage, and then immediately after begin holding L. Note: Having a white flag on the upper right corner of a stage icon at the select screen is how to tell if you have completed a particular stage.Select non-unlocked stages
If you press a direction on the D-pad and A on the same frame when choosing a stage for a battle, you can choose the next stage over, even if you have not yet unlocked it. Additionally, if you complete the stage, it will be considered unlocked as normal.Unlocking more stages
There are two ways to unlock more stages. The first is to complete every stage you currently have unlocked at least once (as shown by the Flag symbol that appears on the picture). Once you do this, the next stage in order will be unlocked. The second way is to find a stage that is not yet unlocked during a match. If you complete that stage, it will become unlocked.Easily target many opponents
If you want to target many opponents at a time (5, 10, or more), switch your targeting method wildly by using the Right Analog-stick during gameplay (usually moving it in a circle quickly and repeatedly works best). When you are finished, switch to targeting "Attackers," and many people in the lobby will be targeting you -- and you will be targeting all of them. This can make it particularly easy to gain time in the early portions of the game and earn KOs. If you defeat particularly difficult enemies (for example, Lakitus), they can often substantially flood levels. Thus, there is a risk-reward balance when playing like this -- it makes it easy to gain KOs and time, but also can make the game much harder and more chaotic (for you and everyone else in the lobby).Small Mario Bonus
The game does not explicitly tell you what the "Small Mario Bonus" is. Essentially, if you choose to start a match without paying for an extra item, the coins earned at the end (after all other bonuses) will be doubled. Thus, if getting coins is your goal, it is very much to your advantage to not start with an item.