It's a person being stabbed. I have recorded some watermelon being stabbed but it needs more oomph.
This depends a lot what you are trying to archieve with sound (is it supposed to imply power, violence, is it stealthy, do you want to emphasize the weapon used etc), but here are some elements I've used
-Short clip of metal scratching to metal sound.
-Knife stabbed to soil
-Crowbar hitting soil
-Slush stomp impact
-Different fruit/vegetable slashing and stabbing sounds
Ground hitting and slush sounds are especially good for adding smooth body to the sound.
-Clips from water splashes
-Slush stomp sound tail
-Wet fruit destruction sounds (oranges work best imo)
Post-release metal "shing" might work, depenging of weapon type and how much you want to emphasize the weapon.
If it's single stab sound combine impact and release, but if there is pause between thrusting the knife and pulling it out then sequence of impact, vocal fx, release, metal "shing" works well.
These days, it's all about the sound being realistic; which unfortunately means shooting, punching and stabbing the producer. Don't forget to record it though!
Just kidding, but a watermelon doesn't seem like the right kind of consistency; it's hollow and juicy. I think I read somewhere that some foley artist used to use a cabbage. You could try a sack of compacted sand, you should get the friction of the blade then which could be interpreted as crunching or scraping bone. It's always good to have the 'cold' sound of the blade.
Add a low pass filter kick drum sample to it for oomph
I would simply suggest stabbing a thick piece of meat.