That's right. I only have the DSR on gold and I would like to get all of the snipers on diamond. Thje SVU is a pain to get gold and I have to play hardcore just to get 1-shot kills.
If you want to do gold quick then play HC for most of the challenges.
For headshot challenges, almost all weapons are 1 shot kills from medium distance in. So just need to learn to aim high.
For 1 shot kills, well this just needs no explanation.
Same for the Assault shield.
The only ones I would say to do in core would be the specials (combat knife, bal knife and xbow) sans shield.
Then for some of the extended challenges (bloodthirsty medals etc), its split.
But for the basic challenges play HC for an extremely fast diamond grind.