I have not yet played this build this season (and am not a Rell main) but I had some success with Eclipse Rell in Season 13 and already tried out Bloodsong on her. It was fun. That is why I decided to combine the two with Press The Attack. Still a work in progress tho :)

If you want to play this I recommend playing Rell with a Premate Samira or Nilah. This is what I played with a friend of mine and it works really well most of the time.

The main goal is to hit as many enemy with your Ferromancy - Crash Down / Mount Up and trade using the dismount-shield. If you have your Flash up and decided to take Hextech Flashtraption, you can wait in a brush and engage using your summoner Spells if neccesary. Keep in mind that you want to AA (autoattack) your Target after engaging in order to trigger Break the Mold, Bloodsong and Press the Attack.
If you only want to go for a short trade together with your ADC you can keep your Shattering Strike and Full Tilt for the disenagae.

For an engage you can extend either Shattering Strike or Ferromancy - Crash Down / Mount Up with Flash. After you land your Ferromancy - Crash Down / Mount Up use Shattering Strike followed by an AA, then use Full Tilt and AA again. If you have Magnet Storm up use it before the first AA (All in: Ferromancy - Crash Down / Mount Up, AA, Shattering Strike, AA, Magnet Storm, AA, Full Tilt, AA)

With Eclipse you have an additional shield for your engage which also comes with a lot of damage which is ideal for the way you want to trade.
Most of the time it is possible to easily fight the enemy botlane 2v2 especially if you get a lead early game.

If you fall behind, try to play safe under your Turret.

If possible try to help your jungler with dragons after reaching level six ( Rell's Magnet Storm and Samira's Inferno Trigger are good for fights in river and dragonpit).