I mean, I still have a MSX2 with a Z80 processor and if you look at any Konami's game made for that computer in its time (roughly between '84 and '90) is amazing the high quality code of those games. I was a kid at the time, trying to learn how to program a computer and still today fascinated me how well made they are, mostly zero bugs or glitches, despite the really complex behavior. What hardware/software tools could they have used to accomplish that quality, which metodology? I know computers are really more complex today but at that time, even a stock control program I made in Basic was plagued with many bugs and was painful to debug. Any light you can shed will be deeply appreciated.
I don't know anything about Konami, so I'm taking an educated guess here. Games on machines like the MSX would have required direct access to the hardware, so that pretty much limits the choice of languages to either C or Z80 assembly language. (There were C compilers for the Z80 back then, for example this one.) I doubt if the games were completely written in C, if at all; mostly likely a lot of assembly code for performance reasons.
As far as platforms go, since the MSX didn't have a hard drive, I would further assume the programs were initially written on a larger Z80 system, perhaps running CP/M. For debugging, the code could probably have been downloaded into a special game cartridge for the MSX that had RAM memory in place of ROM -- this would have allowed the developer to download the program over and over again without burning ROM chips. Debugging (e.g. setting breakpoints) could have been accomplished by replacing the Z80 inside the MSX with an In-Circuit Emulator (ICE) interface.