A good place to start would be spaying the girlie, then waiting 6 weeks and starting again.
You need to make a neutral space inside (you can neutralise a space using a distilled vinegar solution), that is roughly 4ft by 2ft, put them together, supervise them completely (overnight) and see how that goes. That is the safest way and gives the maximum chance of success.
In a large space like a garden they have the opportunity to run and chase and this increases the likelihood of a fight (as you're finding out), also, with her not being spayed, hormones are likely to play a part as well which also makes bonding a huge challenge.