The answer will depend on the pets you have at 25.
I will give you a bunch of places where you can plug in a lvl 8 pet
1. Jade Forst
Use Emerald proto-drake and a flying pet that has cocoon. You can kill the first 1.5 pets with your emerald drake easily. When the emerald drake dies, sub in your level 8 for 1 round. Only thing you need to be careful of is you don´t sub in on the round the snake is ready to unburrow. After a round, sub in your flyer... kill the snake without using cocoon. (ie, take some damage, no big deal). The last pet has only 1 attack which it always does as soon as it is available, unless it´s heal is available, his attack is on a longer cool-down than your cocoon.. in other words, you should never get hit. Just cast cocoon as soon as you see his attack is off cd. Even if he heals, his next attack will still be blocked by the cocoon.
2. Kun Lai
Use Crimson Geode... the enemy soars on the second round, so use amplify magic on that round, when he lands, just kill him. The rabbit comes next.. just make sure you use elemental bolt before you die ( launches a rocket in the air). When your geode dies, bring in Ameshyhe spiderling and ´brittle web´ the rabbit and watch him kill himself. The goat is next. cast brittle web again, then poison spit (dot). The goat will kill you at the end of round 2, bring in your level 8, and the goat will kill himself using swarm, or if not, the poison spit dot will kill him.
Use Crimson whelp, when he dies, sub in your level 8 for 1 round, then KunLai runt. When you get to the third pet ( who has a bunch of avoidance spells). just spam your 2 button until it hits, then 3, then 1 It sometimes takes awhile for 2 to hit, but once it does, fight is over.
4. Water Spirit
Any flyer that has soar. Soar above the 2 attacks, and usually you can kill the first pet without your flyer taking any damage. I usually then bring in a Hyjal bear and kill the second pet. On the third pet, use the bear stun, sub in your low level, then sub right back out. Just make sure you don´t sub in onto one of the delayed attacks.
5. Kars Wild
Darkmoon Tonk - Use minefield on second attack ( he will burrow). Just kill him using your 1 button. When the flyer moves in, it will trigger minefield. Then immediately use Ion Cannon. Disgusting ooze will finish off the flyer, then wittle down the third pet, make sure you have 2 dots on the enemy pet as the disgusting ooze dies (sometimes I have to pass just to let him die). you sometimes have to take 2 hits, so maybe a lvl 8 pet is iffy here Avoid using a critter, flying is best.
6. Vale ( I use frosty, might not be available)
Hyjal bear to kill first pet. As soon as first pet dies, switch to frosty. Use Blizzard, then ice tomb.. then just kill. The ice tomb will trigger a pet switch, just use frosty and bear to kill everything. I usually let frosty die, then swap in my low level, then switch right away for the bear to come back.
7 Earth Spirit ( requires darkmoon rabbit)
Dodge on first round, use bleed attack, then burrow. Sub in low level pet after rabbit dies for one round. I use darkmoon tonk as last pet.. The third enemy pet burrows on his second turn. I use ´lock-on´ during that round and then whack him when he comes up.
I just use Mojo and his main #1 attack almost the entire time. The 2nd pet will do a burrowing move on his second turn, sub in your lvl 8 pet then and sub right back out.. bring Mojo back and finish off the second enemy. Mojo usually almost kills the third pet, but have a mechanical pet in your third slot for 1 or 2 mechanical attacks. On the second turn, the 3rd pet will burrow, so Mojo can get in a heal if you want him to live.
that is basically 8 fights where you can get a lvl 8 pet into the fight without much trouble unless you have some really bad RNG. Kars wilds in particular has a bunch of 25% chance to stun attacks which can mess things up.