You may need the help of these items:
A powerful weapon with freeze-slash mark(Another option is an ice staff with many charge),
A powerful shield with armor or wind mark(It can reduce damage caused by arrows), it's better to have a heavy mark,
A weakening staff and a poison staff with MANY charge,
A Aery tribe friend with tier 2 or higher.

The wiki tells us that Bicorns can be spawned at 450+F. You also can found them in Speed-Theme monster house. If you found a Bicorn that are not spawned by monster house, you can use a grimoire of vacation to another monster. It will increase the spawning rate of Bicorns.

Let Aery standing in Bicorn's sight, she will attract Bicorn to shoot her. Then you can get closer to Bicorn. You also can waiting in a tunnel, and back to a thick wall.
When you are next to Bicorn, you need to increase her love to 3 heart or higher. Then you can use freeze slash or ice staff to freeze her. You should use weakening and poison staff to the ice pillar, the magic bolt will be reflect from ice and hit you. After that, you will got 3 debuff. Then you can trying to defeat her and praying for your luck. Making friend to Bicorn is cruelly hard.