I can do the uni-knot all day but cannot get the Slim Beauty knot! I have 30 pd. braid to a 12 pd. mono leader but just cannot get the line to make the figure 8 to start the knot

. Will try again tomorrow......
The figure 8 is a simple overhand knot partially pulled down.
Guessing your matching line diameters but keep in mind that the ultimate breaking strength of the line is determined by knot strength of the weaker of the two lines.
There are three advantage of braid do to its small diameter per unit strength. 1) increased reel capacity (smaller reels) 2) minimum line stretch (better hook sets) 3) less resistance in the water (requires less weight to obtain given depth)
Unfortunately, a large percentage of reels out there are not all that braid friendly leaving people to recommend selecting braid by matching diameter over strength.
I run #14 braid with #20 leader on my light tackle stuff.
My #14 Suffix 832 braid tests out at #25+.
When combined with #20 mono tied with a 95% knot strength (slim beauty), the working strength of the finished line is roughly 19 pounds. Plenty for any inshore fish I may stumble on to.
In your case, most #30 braids test in the mid to upper #40 range. Combined with your #12 leader, you end up with a working line strength of just #11.5