The problem here is that you are assuming that the "Gensuru's fist would do 100 (base) + 60 (Materialization mod) = 160 damage" is relevant to this case, when it's clearly not: Genthru doesn't punch, or does damage with his own physical capability here. He grabs his opponent and activates little flower. In such a case, only little's flower's own damage is relevant, and how much energy one needs to defende against it.
Thanksfully, Genthru gave us a clear and definite answer to Little flower's power:
This means that Genthru himself needs to become immune to little flower, and that only 25% of his own aura is enough to do it. This means that a reinforcement user with the same level of aura would also only need 15% of his own aura to become immune to it (KEN does 50% to the whole body).
That's... a pretty damn sucky technique TO FIGHT, because it doesn't take into account Genthru's own physical ability as a baseline.
However, it's a great ability for BULLYING and BLACKMAIL, which was the whole point of the ability.
EDIT:
I see, it seems you do not understand what is an Hatsu.
An Hatsu is taking a base skill, and training it so much that it exceeds the normal level it should attain.
Let's take Gon's ROCK for example. Gon has two restrictions on it: he needs to say 'first is rock', and he needs to put himself into a 'starting' position. However, both restriction are irrelevant to the fact this is a hatsu:
If say Gon's "KO" is using 100% of his apparent aura at the normal 'level 5' Reinforcement Gon arbitrarily has, then "ROCK" will use 150% of his apparent aura (taking aura from his reserve aura) and his mental state would be such that it would be 'Level 8' reinforcement.
The notion of 'Hatsu' means you take a skill, and trains it up until it is stronger than it should.
And, YES, Gon's Rock has it's vulnerabilities: Gon still needs to put all of his aura into his fists, and it's still ridiculously draining.
In Phinx's case, there is absolutely no rule about him needing to stay still. However, there probably is a rule about him using Ko during the whole 'charge-up', which means he is extremely vulnerable if the enemy manage to hit him.