why would he not pull out his kazekage puppet when its a counter to minatos ftg seals? :erm:
Sure he can stay in his hiruko puppet for a bit and try to stretch some things out but why risk that when he can start off with his best puppets and use there full potential the moment he can seize it?
I will give you your best arguement which will allow sasori to actually be able to defeat minato. You can add in proof and extras to it as you see fit.
Firstly, Sasori
WILL WANT TO START IN HIRUKO.
This is because, hiruko eliminates all his short comings. He doesn't have any visible finger movement or chakra strings (predict attacks). His heart isn't visible (weak spot). He isn't vulnerable to taijutsu. And, Most importantly, hiruko has heavy armor.
In hiruko, he can assess minato's abilties and techniques. Hiruko alone would single handedly allow him to see FTG and potentially FTG v2 in action. Sasori would then pressure minato with hiruko's traps and tail etc.
None of this will hit minato. But that isn't the point. Sasori has gained valuable intel on the fight, and now he has reduced his opponents stamina just a wee bit.
Now, if minato either;
Destroys hiruko, or summons bunta
Sasori's next move would be to activate his human self as quickly as possible (this is to offer some form of extra defence for his human body through the chain, blades, and fire. Especially giving him time to even do a summoning as minato won't be able just blitz rush through that and would have to take at least a moment to formulate his next move). Minato won't necessarily know about his heart weakness, so the defence of human puppet is just enough for sasori to survive any devastating attack during what should be his next move after leaving hiruko.
Next Step
SUMMON ALL 300 puppets. Minato is a shinobi of quality. None of sasori's puppets are of the quality of a shinobi like minato. Sasori
needs to use quantity over single quality here. With this, sasori needs to surround himself with a few puppets, while using a few hundred of them to start pressuring minato and bunta. The puppets covering him are enough to counter incoming kunai. Alongside his human body, these 30+ puppets surrounding him can be used to keep minato away from his real body. This is a very valuable strategy as i cannot stress that minato doesn't know about his real body assembling or his hopping heart. If minato somehow bypasses all this defence, sasori can hop hearts just before/during a hit. His puppets will be close enough and he can quickly counter attack to get minato to FTG away etc. the 100-250 other puppets all put pressure on minato and bunta while they're at a distance. Sasori weaves them around toad bullets to inch them closer, even if he has to move himself with his army of puppets surrounding him. His job here is to get rid of bunta and continue to wear down minato. This will be hard to almost impossible, and might not even be likely.
If sasori manages to get rid of bunta, good on him. Minato lost some chakra/stamina summoning bunta and has been significantly drained from this drawn out fight with all these puppets and sasori's hopping heart. When sasori's down to less than 10 puppets, it is time to bring out kazekage. This way, he still has some defences while kazekage is coming out and summoning IS for use. Now, Sasori has far fewer puppets to control so has better control over them, has some puppets to hop hearts to(keeping them around him for defence/obstacle) and has kazekage on the attack.
If bunta is gone by this point, that's very good as kazekage's IS has a far easier task now. If not, well shit, sasori's lost. But, If bunta is actually dead, This is where magnetism becomes useful. There'd be a litter of kunai around the field by now and sasori can manipulate them to whereever he wants and can Also use some IS for extra defence/flight. Eventually, minato will start to get warn down and his speed and reactions would slow ever so slightly. Sasori's chances of getting a scratch in would be increased greatly, and the chance of minato destroying kazekage puppet would be reduced to almost nothing (bunta tired/dead. Minato possibly used 2+ Rasengan, a few 10s of FTG etc, so his stamina is lowered). Since sasori has the speed, scale/power(bunta and other summons), and pupeteer disadvantages, he needs to use a strategy that'd best cover his bases. Especially against that speed part, it's the real killer... and maybe sasori can win High diff... Maybe... Assuming he's not killed right out of hiruko before he can even start up human body that is.
This way, sasori's strategy is to keep his defence as tight and as strong as possible and have puppets to quickly hop hearts to as soon as he gets out of hiruko. He needs to eliminate his taijutsu and heart disadvantage from the get go and wear out a speed demon like minato before pulling out his trump card. Bring out Kazekage would simply just let a full stamina minato form a quick strategy to destroy his best puppet, and from there it's all downhill. Sasori would have no chance of winning what so ever.
This is a chest battle, not some muscle flexing I have more power than you. Sasori needs to get to weed out minato's tips and tricks while keeping one of his best ones a secret for as long as possible. It's true kazekage is his best bet in this fight, but against a fast, strong, smart opponent who's full strength would just be suicide.
Something along those lines at least.
Hiruko to Sasori + 300 puppet army to kazekage( best for last - Ruin FTG offense again )