I don't think there's any reasonable closed-form function that will solve this problem, but there are algorithms that will calculate the probabilities for any number of dice of any number of sides.
For a single round, we can simplify the problem a little bit by
reducing the results of all the rolls to just one variable.
That is, starting from zero,
add the dice rolled by A and the "multiplier" $m,$
then subtract the dice rolled by B and the "multiplier" $l.$
If the result is positive, A wins.
For example suppose A rolls four dice whose largest numbers are
$4, 4, 6,$ and $13$ (where you get a fair $13$-sided die is another question, but let's suppose you can) with "multiplier" $6,$
and suppose B rolls three dice whose largest numbers are
$4, 12,$ and $12,$ with "multiplier" $5.$
Then you can go to http://anydice.com/ and put this "program" in the input text window:
Click "Calculate" and select View "Export" and Data "At Least".
Look in the output window where you see the line
That tells us that A has approximately a $52.8669482283\%$
chance of winning each round.
You can get a few more digits in the answer by omitting
>0 from the "program." The output can get quite long in that case and you may have to scroll down to find the line that starts with $1.$
But the last few digits in that format are not quite correct, anyway; you can get an idea of how accurate this calculator is by selecting Data "At Most" and scrolling to the bottom of the output window. The probability there should be $100\%$ but it typically is not exactly $100$ for the examples I have tried.
If "multiplier" actually meant something to multiply the outcome by rather than something to add to it, the "program" is
Another on-line calculator is http://topps.diku.dk/torbenm/troll.msp,
but it works a little bit differently
and I have not experimented as much with the programs there.
Once you have your probability for a single round,
you can formulate the game as a binomial random variable $Y$
with parameters $2n+1$ and $p$ (where $p$ is the probability that A wins one round).
For example, suppose there will be $25$ rounds ($n=12$).
Then the probability that A wins is $P(Y \geq 13),$ which we can also calculate on-line:
Wolfram Alpha gives the answer $0.614021,$ that is,
A has a $61.4021\%$ chance to win the game.
If you want to set up all these calculations yourself rather than use the on-line resources, a reasonably simple program will do it,
or even a computer spreadsheet with appropriate formulas.
I could go into more detail on that if that's desired.