I played through this game a few times aiming for speed (casually though), so I can offer some advice.
First of all, don't even bother with Baby forms, they offer absolutely no good skills and are replaced after the first (real) boss, so the only point you'll be using it is on said boss. I suppose this is pretty obvious if you've played the game before, though.
After a Roll/Dash jump, you can keep your speed by firing your blaster repeatedly before Ellie starts to descend. You can even jump Golems and Giant Crabs by doing this. Manipulation for blaster energy drops or trips to save rooms would be necessary, since you can only fire 48 shots off the bat from a full meter.
As far as which build to go for, I recommend sticking with Ballista (the first Teenage form) and using Resistance of Midnight. It's without a doubt the best skill to fight bosses and big enemies with.
If Ballista gets to level 10 and masters Resistance of Midnight, it might be worth looking into if it'd be better to switch to Sentinel (the first Adult form) or sticking with Ballista. At a glance it seems to do as much damage as Ballista does at 10 with Fire Attack Affinity II, but I could be wrong. If for some reason Ballista gets to 15 and masters Fire Attack Affinity II, there's no reason to not switch to Sentinel, since it has better attack and gets Attack Up II at level 5.
Speaking of Fire Attack Affinity II, if you end up getting it, I would advise equipping it. The amount of damage it adds is barely noticeable per shot, but it certainly adds up against bosses. Even if you don't go out of your way to get it, you'll probably end up getting it anyways due to the monster waves and the occasional road blocking Golem you'd want to clear faster by using Chomp.
Some of the annoying enemies that tend to block the road can be quickly bypassed by launching them upwards or by using a Burst Cancel and running by them when they're down (Burst Cancel even works on Giant Crabs). Of course, this doesn't work on Golems, since they don't flinch.
Merely as a guideline for about how long the game is, my last playthrough with unintentional overleveling (15 Ballista, 12 Sentinel) due to an abundance of random item drops was
1:51 1:50 game time. Since most of the game is spent running around collecting new abilities rather than fighting bosses, letting Chomp consume everything shouldn't even save much time (game time anyways, if even that), so it should be able to be beaten easily by a very large margin just by knowing where everything is and knowing the best gems to pick up for the mandatory 2750 gold (2612 with the alternate outfit, I think) you have to spend in Ethanica.
Edit: Just pointing this out, this game has no potential for sequence breaking, as if you try to get powers out of order, the power just isn't there. Same goes for scenes, you can't do anything out of order.
Failed sequence break:
http://www.youtube.com/watch?v=9por5IUP6yo
Dash gate bypass (useless due to former):
http://www.youtube.com/watch?v=MC5Qmzd4KuQ
