[RPG] How to deal with a Batman PC as a DM

It is a standard action to administer a healing potion to an adjacent player, even an unconscious one. This will allow the fallen PC to pop back up. Its not much worse than a heal check (also a standard that required adjacency) in that it may (or may not) cost a minor to pull the potion, but at least its a RAW option.

You can also expend a standard action to make a DC 10 heal check to allow a character to expend their second wind (they don't have to be unconscious).

My real suggestion though is to have one of your characters MC healer. Its a bit of a feat investment, but in this party it is sorely needed. It is a fairly limited solution, but if your other characters have some personal healing effects (also a must in this situation) it can be saved for such a moment when a comrade has fallen and it is inconvenient to try another solution.

From looking at your group I would put my chips on the problem just being that the psychic warrior is optimized while the rest are not.

In a brute force manner you could do the following, but I do not recommend any of these:

1. Increase the CR of encounters. Doing so would make it more challenging for the Psychic Warrior, but would run the risk of making it too hard for the other characters.
2. Decrease the CR of encounters. Doing so would allow the other characters to be able to handle things like the psychic warrior is now, but the psychic warrior would just be stronger in comparison.
3. Nerf the psychic warrior. Doing so would make him more on par with the rest of the party, but the player would likely feel targeted and punished. In my opinion this is never the answer.
4. Buff the rest of the party. Doing so would put everyone on par with the psychic warrior, but if that player is the type of gamer who min/maxes and optimizes he will want the buffs too, and would likely feel left out.

Since none of those solutions will solve the problem in a fair way I would recommend is tuning the game to give everyone a chance to shine. A few examples:

1. For the ranger include times when an enemy needs to be tracked, or include the ranger's favored enemy in encounters more often.
2. Include enemies that are resistant to physical attacks, either through high AC or DR, that the wizard can better handle. And at higher levels include encounters with a lot of weaker enemies that can be taken care of with area spells like fireball.
3. Include more traps for the rogues to take care of, or make sure that there are enemies that can be sneak attacked. Or if the rogues are more charismatic you can include social encounters.
4. Include undead for the cleric to deal with using turning or positive energy (I assume he is a good cleric).

As a final note, your campaign is still in the very early levels. Eventually you will likely find that the Wizard and Cleric will start outshining every one else, that is just the nature of D&D 3.5. This is an idea called Class Tiers, which says that some classes are just better than others because of their raw mechanical power and versatility. This system does however rely on all characters being played at an equally optimized level, so if the one character is much more optimized than the others then this may not apply.

TL;DR Don't just try to buff or nerf the encounters or the party. Try to include situations where each character can shine, including outside of combat. The problem may just resolve itself in time as some characters become more powerful than others at higher levels.