I did this in the first event. I'm almost level 200 now and the two things I learned you need to be sure of if you want to get through heroic mode quickly with little gems are:
1. Use your friends to your advantage once all of your useable knights are low on health.
2. Pay attention to the amount of experience you need to level up.
If you make sure you use all your knights health and find you're about to level up plan your roster accordingly; it's a good idea to be careful not to waste a friend when you're about to level up.
In the first 8 stages as long as you pick a friend with strong armor that counters the boss of that stage you should be able to get by using 1 knight with little health with that friend. You can find a list of the bosses and their elements on this wiki.
The later 7 stages are very similar, however as you may have noticed.The bosses of each stage become much stronger as you go further.
>>>This may require you to use a knight or two with full health and 1 friend to beat some stages especially the Zephyr Plateau and Haunted citadel.
Once you make it to the Final stage you should have as many as 3+ weeks left to collect the final 8 shards Be sure to use two very strong friends when doing the Kingdom of darkness. The elements are best to contain fire/anything and fire/water.
Good luck!