weighted cross entropy for imbalanced dataset - multiclass classification
If you are looking for just an alternative loss function: 

Focal Loss has been shown on imagenet to help with this problem indeed. 

Focal loss adds a modulating factor to cross entropy loss ensuring that the negative/majority class/easy decisions not over whelm the loss due to the minority/hard classes. 

I would look into using that is it seems to be really promising. 



Link To Focal Loss Paper:  https://arxiv.org/pdf/1708.02002.pdf