If you want to use magnus, buying ASV and then using that to buy EaseMyTrip voucher is the best option. You will get 5x ER which is ~ 25%.
IMO, if you have Infinia points, best option is to use points to book hotel through smartbuy. Price is same as directly booking through cleartrip but you will not be able to use promo codes. 70% using points and rest 30% using Infinia which will give you 33% back as points.
If you don't have any points, then go to cleartrip through HDFC smartbuy. It will give you 33% back as well but not sure if it allows using promo codes.