I'm no expert at VF5, but I've been plowing through quest mode easily. Sure, you can spam Y,Y over and over, but after the Dan ranks you need some more tricks. These moves will help you slaughter the upper ranks!
I used the default button layout w/1 exception - I had A perform P+K instead of a 2nd P button.
Start every match with forward, forward + A, A. This is a 2 hitter that does pretty good damage and has a high hit priority. Feel free to spam this from a distance as enemies begin an attack. Also works wonders when getting up.
Follow the 1st combo with RT, Y. What's nice about this move is it seems to also hit a lot. If the enemy dodges the 1st hit, #2 often hits anyway and always knocks enemies down. Use this to back up a bit. There is a slight delay when starting the RT attack, but often opens the enemy's defense for an easy hit.
Using the simple combos back-to-back, if all 4 hits connect, will take almost 1/2 an enemy's life bar. Don't worry if enemy blocks 1st combo, they usually fall for 2nd combo.
I've also found that air strikes on a downed opponent always hit if enemy bounces off a wall 1st. Use it risk free if you trap someone!!
Finally, if your opponent blocks all 4 hits and counters you either between or after all hits, try the forward,forward,A,A from a distance as an enemy begins a move. And this time follow up with a bunch of regular punches instead of RT, Y. Likely, they will get punched a lot instead of throwing/countering you.
1 other simple combo if forward +B, spam Y. It's just a knee plus some juggle punches, bit it's slightly quicker than the RT combo. It's also great for getting ring-outs and wall-bounces (which opens free downed enemy hits). I prefer the RT set overall, but this is good if you want to create some spacing.
Enjoy doing massive damage!!
I see some people have been checking this thread out, and I screwed up! I edited, so what you might have read as LT, Y should now read RT, Y instead. Sorry for the confusion. If anyone has tried the old combo and didn't see the reason, try the revised combo of RT, Y now. BIG difference!!
Seeing as how I've got just under 20% left in my last arcade for 100% achievements, here's 2 final bits I found. Remember that I changed the A button to be P+K instead of just a punch....
Here's a nice combo for kyu and weaker Dan enemies. Start with forward, forward A, A, continue to hit A x2 for a juggle combo, Y x2 for more juggling, end with RT, Y. This will take about 60% of your opponent's life bar. You can try this on anyone, but higher Dan and up will block a lot and might counter or throw. The weak opponents won't block, and doing this combo twice will win the round in under 8 seconds.
The other tip was found on accident against elite opponents. If they're blocking everything and nailing you with counter hits or throws, tap forward x2 but do nothing. If you see the enemy flinch or try to hit you, then this will tell you that you can tap forward x2, then do the forward x2 + A x2 plus whatever after. This might seem cheap, but against human opponents in fighting games faking moves is a solid tactic.
If any of these tips exist in any guide or forum, I apologize. These are things I stumbled on while messing with weak kyu enemies.