Can I use neos fusion to fusion summon elemental hero grandman in yugioh?
No, Neos Fusion can only be used on Fusion Monsters that specifically list 'Elemental HERO Neos' as one of its two fusion materials, the word 'list' is the important bit. It can't be used on a fusion monster that simply 'could' use Elemental HERO Neos without specifying it by name, like anything that could use Light-attribute monster, or the 'HERO' monster requirement of Elemental HERO Grandman. Also...note that Neos Fusion does not Fusion Summon the card. While it does ignore summoning conditions and can therefore get around 'must be Fusion Summoned' monsters, note that Grandman's ATK is determined by the levels of its Fusion Material - if it manages to get on the field without actually being Fusion Summoned then it has no material to refer to and its ATK will be 0.