dumpprivkey
In Electrum, make a new Electrum wallet. Then go to Wallet > Private keys > Sweep and enter your private keys into the box and follow the instructions. That will sweep the Bitcoin from your Bitcoin Core wallet to your Electrum wallet. Note that it is better to sweep private keys rather than import.
As an extra note, if you sweep your bitcoins then it is a transaction that takes place so it does cost your a regular TX fee to it.
Normally these are quite small in value though.
If you follow achow's system it'll probably be the fastest and more effective.