Get through the highest difficulty, Non-Stop Climax. If that ain't basically it, I don't know what is. 
Slow-to-activate?
Really and truly, the combos are only as "slow" as your hand-eye coordination allows you to have. Want the combos to come out faster, then your fingers may need to get a bit more training in their reflexes.
But then again, if you do want something a bit more deliberate and powerful, the game's got you covered. How do Fire Claws and Tonfa Bazookas sound to you?
You can press P,P,P,K,P too fast and the combo doesn't activate. Does that in training, does that in combat.
Developers don't seem to get that the only people that care about Ranking, Difficulty, and Combos are a very niche group of nerds with nothing better to do. The larger group really doesn't care, want a powerful and competent main hero without feeling like you've SNOBBED US LIKE WE'RE SOMEHOW INFERIOR FOR PREFERRING SOMEONE WITH SOME ACTUAL POWER, and enjoy the settings and story since the devs hide secret missions and areas and spend a lot of time making you watch cutscenes and travel.
And you still really can't dodge while you're trying to input these long combos, and if an enemy decides to go nuts on you while you're putting it in and it's wanting a certain timing, well you're just SOL.
Brutal Legend had longer combos, but they weren't necessary. In addition, it had an all-range, damage stopping Guard. This made you focus on knowing when to pull off combos instead of going in headlong. And in the meantime, you had plenty of shorter inputs that, though not as powerful, had effect. Stop an enemy in its tracks, daze them, set them on fire, etc. Brutal Legend would have been the best Action game if it had some more of the genre's traditional elements (like a freakin' lifebar) and the RTS hadn't gotten in the way.