Two (sets) come to mind for me immediately, though one is semi-optional (you have to fight, you don't have to necessarily win, but you can't just lose either).
Al-Van in Super Robot Wars J. Much like the later entry on this list... the secret is he's a boss from later in the game. At stage 20 you fight him. Al-Van's personal stats as a pilot are on par with what enemies of that point in the game should have. His robot's stats on the other hand, are roughly what they are later in the game when you fight him to the death, at stage 38. I think there may be a couple upgrades' difference for the robot but that amounts to a few hundred more points of armor and damage to weapons as well as a few points of dodge and accuracy that's more overshadowed by the level gap for the pilot, honestly. Al-Van hits like a truck, has a shield (chance to reduce damage to 40%, always used if defending and a chance to use when counterattacking/being counterattacked based on how much higher his Skill stat is than the other unit [as a boss he'll have higher skill unless you deliberately pumped that stat]), a strong attack that can one-shot your dodgier guys if they eat a crit (skill influences crit rates), and once the fight's been going a while and it hits 120 morale (trivially easy) the real fun starts. Two things activate at that point, a barrier that reduces all final damage by 1200 is added in, which is calculated after the shield, and a 50~% (skill gap influences exact chances) to just stop time and dodge an attack, regardless of what the hit rate says. The only way to circumvent the dodge is to use the Spirit command, Strike which forces a 100% hit rate and overrides everything else (including when the enemy is scripted to use Flash which is a 100% dodge). Al-Van also regenerates 30% of his energy every turn so a strategy of bleeding him dry doesn't hold up. It's a particularly rough "get serious" moment because when does he show up? As a third wave of enemy reinforcements. Most players not expecting this will have spent a lot of their SP on the previous enemies and then be caught unprepared for the ambush.
Super Robot Wars OG2 had a trio of semi-optional bosses on stage 15. They are endgame bosses (you fight them for real on stage 38 out of 43) with the same stats for their units as the endgame, but lower pilot stats (they're still overleveled for the stage) so you have an actual chance to hit them. Emphasis on chance. Your have four units for the stage and they're new with no prior chance to upgrade (or neglect, tbf) them that it's purely up to your stubbornness and awareness as a player if you want to fight them and if that fight is winnable. The mission objective is to escape, but you can fight and beat them. There's a big object that gives hp and energy regen, as well as defense and dodge bonuses. But you frankly have no chance of winning if the enemies position on that terrain themselves (Literally no chance, the hp regen will be greater than your damage output even if all your attacks hit). You need to control the AI and pace yourself for a marathon with knowledge that you WILL not be able to just plop units down and keep them there, since you have no resupply unit and have to move units into your battle ship to reload ammo so they'll be able to finish the fight. Fortunately the enemies and your party do talk before the fight starts and the bosses each call dibs on one particular unit, so you know who will be attacked by who and in theory won't have a string of just getting hammered more than you're able to deal with. It's about as pure a test of player knowledge as you can get in a game like that by making it default units only with no chance to dump all your resources into them before the fight or be screwed by just not using the unit and suddenly having them be a mandatory deploy. (Though if you kill the wrong enemy first he'll retreat and the other two will go with him, denying you the rewards for defeating them which are some really, really good items)
Two (sets) come to mind for me immediately, though one is semi-optional (you have to fight, you don't have to necessarily win, but you can't just lose either). Al-Van in Super Robot Wars J. Much like the later entry on this list... the secret is he's a boss from later in the game. At stage 20 you fight him. Al-Van's personal stats as a pilot are on par with what enemies of that point in the game should have. His robot's stats on the other hand, are roughly what they are later in the game when you fight him to the death, at stage 38. I think there may be a couple upgrades' difference for the robot but that amounts to a few hundred more points of armor and damage to weapons as well as a few points of dodge and accuracy that's more overshadowed by the level gap for the pilot, honestly. Al-Van hits like a truck, has a shield (chance to reduce damage to 40%, always used if defending and a chance to use when counterattacking/being counterattacked based on how much higher his Skill stat is than the other unit [as a boss he'll have higher skill unless you deliberately pumped that stat]), a strong attack that can one-shot your dodgier guys if they eat a crit (skill influences crit rates), and once the fight's been going a while and it hits 120 morale (trivially easy) the real fun starts. Two things activate at that point, a barrier that reduces all final damage by 1200 is added in, which is calculated after the shield, and a 50~% (skill gap influences exact chances) to just stop time and dodge an attack, regardless of what the hit rate says. The only way to circumvent the dodge is to use the Spirit command, Strike which forces a 100% hit rate and overrides everything else (including when the enemy is scripted to use Flash which is a 100% dodge). Al-Van also regenerates 30% of his energy every turn so a strategy of bleeding him dry doesn't hold up. It's a particularly rough "get serious" moment because when does he show up? As a third wave of enemy reinforcements. Most players not expecting this will have spent a lot of their SP on the previous enemies and then be caught unprepared for the ambush.
Super Robot Wars OG2 had a trio of semi-optional bosses on stage 15. They are endgame bosses (you fight them for real on stage 38 out of 43) with the same stats for their units as the endgame, but lower pilot stats (they're still overleveled for the stage) so you have an actual chance to hit them. Emphasis on chance. Your have four units for the stage and they're new with no prior chance to upgrade (or neglect, tbf) them that it's purely up to your stubbornness and awareness as a player if you want to fight them and if that fight is winnable. The mission objective is to escape, but you can fight and beat them. There's a big object that gives hp and energy regen, as well as defense and dodge bonuses. But you frankly have no chance of winning if the enemies position on that terrain themselves (Literally no chance, the hp regen will be greater than your damage output even if all your attacks hit). You need to control the AI and pace yourself for a marathon with knowledge that you WILL not be able to just plop units down and keep them there, since you have no resupply unit and have to move units into your battle ship to reload ammo so they'll be able to finish the fight. Fortunately the enemies and your party do talk before the fight starts and the bosses each call dibs on one particular unit, so you know who will be attacked by who and in theory won't have a string of just getting hammered more than you're able to deal with. It's about as pure a test of player knowledge as you can get in a game like that by making it default units only with no chance to dump all your resources into them before the fight or be screwed by just not using the unit and suddenly having them be a mandatory deploy. (Though if you kill the wrong enemy first he'll retreat and the other two will go with him, denying you the rewards for defeating them which are some really, really good items)