1. Introduction of the Game:
Door Kickers: Action Squad is a fast-paced, side-scrolling action game developed by PixelShard and published by KillHouse Games. In this thrilling title, players take on the role of SWAT operatives tasked with resolving hostile situations in various environments. With an emphasis on quick tactical decisions and reflexes, Door Kickers: Action Squad provides a mix of raw action and strategy, making every mission a high-adrenaline experience.

2. All Characters and Levels of the Game:
The game features a selection of unique characters, each with their own special abilities and gear loadouts. From the Assaulter, who packs heavy firepower, to the stealthy Stealth, who can take out enemies silently, along with breaching options like the Shield and the Recon specialist - players have diverse strategies to choose from. The game spans over several levels, divided into multiple missions, which challenge players to rescue hostages, disarm bombs, and take out terrorists with precision and speed.

3. What Attracts Users Most to the Game:
Users are most attracted to the game's intense, action-packed gameplay, which requires quick thinking and reflexes. The retro 2D graphics and the strategic depth add a nostalgic charm while still offering modern gameplay mechanics. Co-operative gameplay, where players can team up with a friend to take on the missions, also adds to the game's appeal. The combination of tactical decision-making and split-second action creates a rewarding experience for players.

4. Why It is Popular:
Door Kickers: Action Squad has earned popularity for its engaging gameplay that successfully merges tactical elements with arcade-style shooting. The game's intuitive yet deep gameplay mechanics appeal to both casual and hardcore gamers. Its popularity is also driven by the positive word of mouth regarding its challenging scenarios and the satisfying feeling one gets from mastering the levels.

5. When the Game was Released and How Many Versions Are Currently Available:
Door Kickers: Action Squad was originally released on September 10, 2018. As of my knowledge cutoff in 2023, it is available on multiple platforms including PC, Nintendo Switch, PlayStation 4, and Xbox One. There have been ongoing updates and DLCs that add content and features to the core game, but there are no distinctly separate versions of the action squad game itself.

6. The Most Difficult Part of the Game:
One of the most difficult aspects of Door Kickers: Action Squad is the higher-level missions that require precise coordination, timing, and strategy. Players need to manage limited resources, like health packs and ammo, while facing numerous enemies, traps, and timed objectives. The game's difficulty spikes can be a significant challenge, demanding players to carefully plan their approach to each room and adapt quickly to unexpected situations.

7. Strengths and Weaknesses of the Game:
The strengths of the game include its intense and rewarding gameplay, variety of characters and loadouts, cooperative play option, and satisfying tactical depth. However, some weaknesses might be that the game can be unforgivingly difficult at times, which may deter some casual players. The reliance on trial and error to progress in later missions could also be a point of frustration for some.

8. What Other Games Have Been Developed by the Same Developer:
The same developer, PixelShard, in collaboration with publisher KillHouse Games, is also known for the original Door Kickers game. Door Kickers is a top-down tactical strategy game that provided the foundation for the spin-off, Door Kickers: Action Squad. While not a direct sequel, Action Squad shares its theme and some gameplay elements with the original but translates them into a different genre and perspective.