How should I encrypt a bip39 mnemonic with a password? With no server and only in the browser I'm generating an a bip39 key and giving the user the mnemonic to store some place safe for restoring, I also want the user to be able to login with a password.

1) Could I encrypt the mnemonic with a password?

2) If I do encrypt the mnemonic, which algorithm should I use?
Bip39 has an optional "encryption" over PBKDF2 (passphrase-to-key) used as salt (this allows possible deniability).
https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki#from-mnemonic-to-seed

The weakness there is that BIP39 uses only 2048 rounds during KDF. Which is easy to bruteforce. 

If you store it in a cloud (or similar), I recommend to do a AES-256-CBC encryption of the mnemonic with a strong password and a strong KDF (maybe use PBKDF2 with at least 30'000 rounds).