These are some guidelines to make things easier for everyone. Please follow them, though it's not the end of the world if you don't.
1. Clearly state the thread type plus the relevant character(s) and work(s) in the title and topics.
I.e. "Superman (DC comics, Pre-Crisis) vs. Goku (Dragon Ball Z)" would be a good title and the tags would be "#deathbattle #superman #dc #goku #dragon ball". Other thread types may include respect thread, infodump, debunk, request, etc.
These are some very general and non-absolute rules for deathbattles. These guidelines are what I have found to be most conducive to clear and rewarding debate over multiple years of experience. That does not mean that this is the way that they have to be run.
0. OP is the arbitrator of the guidelines (within reason) for the purpose of their thread.
The person who starts the thread is the one asking the question that everyone else is answering. If they decide that they want to ask who would beat who if their names were literal, or if a normally peaceful character had no morals, or if certain abilities of a character didn't work, then that is the question to answer.
1. Feats are the gold standard of evidence.
This means a direct record of a character demonstrating a quantifiable ability. Names, titles, gimmicks, character statements and other assorted crowd-working can be (and usually are) various levels of nonsense - in either direction. Example: just because Superman is called the Man of Steel doesn't mean he is as durable as steel. The fact that he can smash steel doors and machines with his bare hands does, though, because we actually have evidence of it.
2. Characters have ethics and morals.
Just because a character can technically do something doesn't mean that they will. Characters usually have a certain personality or psychology that leads them to adopt certain strategies when challenged; this may under-utilize or even ignore methods that are unpleasant, have later consequences or are otherwise undesirable or uncomfortable for the character. Example: Batman, due to the trauma of seeing his parents shot while he was young, avoids killing people as much as possible and, even though he utilizes a wide array of technology, refrains from lethal firearms.
3. Neutral universe is default.
Do not assume that one character or another is depowered or otherwise hindered because they are fighting a character from a universe that works differently or because their powers would not work in real life. When you present a deathbattle as being participated in by a certain character according to their media, then their abilities work as they do in that media. However, this does not mean that specific vulnerabilities that those abilities target are projected onto the opponent. Example: Ichigo Kurosaki, as a shinigami, is invisible to the living who do not have a specific ability to see spirits; a character like Captain America would not be able to see him unless specified by the OP. In contrast: the chakra powers of the Naruto world depends on an actual physical "chakra pathway system" that exists in humans in the Naruto world because of a specific event in the past where humans were imbued with chakra; we cannot assume that Captain America has a chakra pathway system that would be susceptible to the ninjutsu of a Naruto character because no such event happened in Marvel.
4. Composite characters are not default.
Do not mix up different versions of characters. Many characters have significantly different backstories, personalities and abilities from one version to another. Some characters can even be seen as roles that are played by different people at different times. Besides the fact that the versions of some characters are literally different people, combining different versions may not make sense due to conflicting abilities or may even weaken the stronger version of the character by burdening them with a vulnerability that a weaker version has. Example: the Flash, while many people refer to him by title, is a role that has been filled by Jay Garrick, Wally West and Barry Allen; each person has a different level of command over their superspeed.
5. Plot does not figure into deathbattles.
Argument based on deus ex machina, character gimmicks, how you would write the fight for maximum drama, plot armor, plot-induced stupidity or other argumentation based on completely removed abstract reasoning should be discouraged. This kind of argument has very little or no basis in the evident abilities of the character and usually does not account for the abilities and concepts introduced in a battle between that character and a character from another universe. Example: just because the name of the series is "One Punch Man" does not mean that Saitama could kill Goku with one punch; Goku has durability far beyond the planet-cracking strength that Saitama has shown thus far.