hmm loki does have some easy ways to win but i cant help thinking that he would go in an engage in some kind of corporeal confrontation and get smashed. Also presuming current loki is still a kid (im a bit behind on fear itself) i cant see him winning this.
As for mangog, physically id say WBH, has much better feats than he does and will only continue to grow in power exponentially throughout the fight. i mean perhaps he could transmute him or something but that doesnt seem likely given hulks past history with transmutation and the direct correlation between his ability to resist such attacks and his anger level.
Regarding the destroyer, Thor with a portion of the odinforce was able to tank its disintegrator beam and id say wbh is more durable than he is based on showings.