“Tiro Certo Metal Slug 4 finals” – the 2024 grand final ended with a double KO on the final boss, forcing a sudden-death round where the timer stops and any hit kills.
I can point you to active Discord servers and YouTube archives.
Here’s interesting content about Metal Slug esports tournaments, focusing on the competitive scene around (not just mobile).
The birthplace of the series still sees high-level play at legendary arcades like Mikado. Japanese tournaments often focus on the "technical perfection" of score attacking.
“Hunter Mode” — a fan-made mod where one player controls the regular soldier spawns (via a second joystick) while the other plays as Marco. It’s janky, but it creates real PvP.