I make no claim to be a premier league bat, but a few thoughts from my side.
I open the batting, so by the time a spinner comes on I'm generally well set. Of course, some teams do open with a spinner, but usually I face them 2nd or 3rd change - maybe around the 18th-20th over, something like that. This means I have some advantage in terms of knowing how the pitch is playing, and have taken the hardness out of the ball. You'd likely be bowling to me when I'm confident, but starting to tire.
In that situation, it's my opinion that spinners usually tend to try too hard. They have waited for a bowl, and then want to attack. They look to vary their deliveries too much, because they think that attacking immediately is the best way to get me out. In fact, the opposite is true. By then, I've seen off the faster bowlers, at their freshest, with the new ball. Honestly, at that point, any sort of spinner looks like a breather, and a chance to push on. The better spinners recognise this, and look to tie me down.
The most important thing is to fix the line you want to bowl me, and pitch it up. You say you're bowling leg spin, flat and fast, so to me, I would aim at middle/middle and off and hope to turn it away a little bit (I bat right handed). Critical is not to pitch it short, by the 20th over I've got my eye in and am keen to hit boundaries rather than run 2s and 3s. Anything short will get the treatment, either cutting or pulling. If you can hold the line and turn it away from me you can then pack the off side field. I'd set a 3rd man, backward point, extra cover and mid off for sure, all on the single. Then set a sweeper out deep to try and cut off the boundaries, and probably a long on. You need to force me to find gaps, to come down the track and to hit over the top.
The best spinner I faced recently bowled left arm orthodox, so not unlike you bowling leg breaks (I'm assuming you're right handed). He bowled 5 balls an over flat, good length, on middle and off. The pitch was turning enough for it to be awkward, so it kept me tied down. I was in the 40s and was looking to push the score along, but he forced me to defend, or improvise. One ball an over was given a bit more air, but on the same line, inviting a drive over mid-off. I hit a slightly shorter delivery through point for 4, and took a few singles off my legs into the gaps. Otherwise, he kept it very tight, and got me out LBW trying to sweep a straight one in his 4th over I think. Good bowling. It was a frustrated shot, gambling that my eye was there to hit it, but knowing that the consequence of missing was inevitable. Looking back, I'd have been better to have tried to disrupt his length by stepping out of my crease to every ball, to try and force him to pitch shorter.
If you've got a googly as well, good on you. Honestly, it will have less effect on me in the 20th over as I should be able to play it off the pitch - defensively or maybe turned through square leg. You're probably better to save that for a new batsman and keep me tied down with the delivery that turns away.
Best of luck!