Is there any chance of the program or a screenshot? (IDK how possible such things for the 82 are...)
Anyhow, i'd recommend leaving away the trailing quotes on your output lines, this way you save bytes!
Also, any reason why there are so many spaces after "YOU LOSE!" and "YOU WIN!" ?

* long post incoming *

For further programs: you may want to add comments what the Vars stand for since we can only guess here xD
So here is what your vars are used for:

A = Score *btw your code says gou got to score at least 150001 since it's > not >=
B = X-Position of the "cursor"
C = Lives
D = Speed (and direction) of Cursor's movements (by default 1)
E = Score of last round (to check if changed)
F = Lives of last round (to check if changed)
G = Counter for the slider (hom many "tries" left)
I = Just for the delay, its value doesn't matter
K = GetKey *you should always declare Variables BEFORE using them
But since the default value for a var is 0, in this case "Repeat K=45" won't make any problems:)



Now let's take the code apart

0->A
1->B
3->C
1->D
0->E
0->F
25->G
ClrHome
Output(1,2,"SCORE
Output(2,2,0
Output(3,2,"LIVES
Output(4,2,3
Output(7,9,"VJust declarations and display stuff (stuff that won't change during the program)

Repeat K=45 or C<0 or A>15000 or G<0Main loopthat got 3 possible criterias to stop:
A) No lives left (lives=-1)
B) Score>15000 (You might want to change that to >14999 or >=15000)
C) NO more tries (wasted all 25)
It would make things easier for you if you made all your future programs end at 0 not at -1

If A!=E
Then
  Output(2,2,"               "
  Output(2,2,A
End
If C!=F
Then
  Output(4,2,"               "
  Output(4,2,C
EndChecks for changes in both lives and score. That's fine but is it really necessary? ^^ You could also just update this every round ^^ (but that might make the screen flicker a bit so just leave it like that)
By the way: since you use "                " a couple of times, why dont you just store it as Str1 and display that? (or maybe Str0 since nothing (zero) is displayed? ^^)

Output(8,1,"................
Output(8,B,"X
A->E
C->FDisplay the ground and the "cursor", fine.
But you might want to add at last the " at the end of a string (so we know there's no more spaces behind coming)

getKey->K
If K=105
ThenThe kay was pressed, will now check if you missed or not

25->GSet number of tries to 25, ok

If B=9
Then
  A+100->AA perfect hit I guess...

If rand>.75
A+150->A Additional points depending on luck

If fPart (A/1000)=0
Then
  A+250->A
  C+1->C
  Output(2,7,"1 UP!
  For(I,1,50
  End
  Output(2,7,"     "
  Output(1,7,"1 UP!
  For(I,1,50
  End
  Output(1,7,"     "
EndScore divisible by 1000: 1up
I like it how you used two lines to make the text move to the top
Also that For( is a good way to add a little delay

Else
  A-150->A
  C-1->C
  Output(5,2,"MISS!
  For(I,1,50
  End
  Output(5,2,"     "
  Output(6,2,"MISS!
  For(I,1,50
    Output(6,2,"     "
  End
EndCode part if you didnt perfectly hit it
Why do you put the Output(6,2,"     " into the for-loop  It will only cause a much longer delay and maybe make the screen flicker...

For(I,1,10
EndAnother little delay
You may put this in one line, you know...? like ":For(I,1,10):End" that's good for readability of your source (as ling as the for-loop i only for delay)

B+D->BMove the target again (speed and direction=D)

If B>16
Then
  16->B
  0-D->D
EndMustn't leace screen on the left side
Just do -D->D here (the negative-sign, not the subtract)

If B<1
Then
  1->B
  0-D->D
EndMustn't leace screen on the right side
same as above

G-1->GYou should put G-1->G into the two conditions above (leaving screen) instead of putting it here Else You won't have 25 chanzes - it's more like the target is gonna move a total of 25 times... (here the counter for left tries (G) gets reduced by 1 EVERY round no matter if it touched the borders or not, that's what you could call a bug)

EndEnd the conditioned block for Key is pressed

If C<0 or G<0
Then
  Output(7,1,"                "
  Output(8,1,"YOU LOSE!       "
  Pause
EndI lost...
ok ^^

If A>15000
Then
  Output(2,2,A
  Output(7,1,"                "
  Output(8,1,"YOU WIN!        "
  Pause
EndSame as before: This is triggered at 15001 instead of 15000, use >= here or simply use 14999 instead

ClrHomeok, program is going to end

I'm not sure, but isn't an End missing here for the Repeat K=45 or C<0 or A>15000 or G<0  If you didnt put it because the program is running like this, too: Never do that! it's ik to leave out stuff like " and ) (still should avoid the first) but NEVER leave out a For - it's terrible for readability and it's gonna be a problem once you want to modify the program but can't remember you left out that End...
If It's just me and I made a mistake somewhere, just ignore it

Dont know if you're planning to add this later, but you set a parameter for the speed without ever changing it
And what you most likely didn't think about:
That speed parameter might be a problem since it' imposible to score a "9" at any speed paramater D where iPart(8/D)==(8/D) is not met since B is gonna change like that:

Speed	how B changes
1	1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1-1-2-...
2	1-3-5-7-9-11-13-15-16-14-12-10-8-6-4-2-1-3-...
3	1-4-7-10-13-16-16-13-10-7-4-1-1-4-...

so at D=2 you can only do it in 13 out of 25 tries...
and for D=3 it's completely impossible ^^

*I didn't try any of the code out myself so I got no idea if it's working...

dont forget the G-1->G or your program should end after 2 tries if you dont press  key (at least I think so ^^)

As in the Str part, TI-82 does not support string manipulation