If I recall correctly you only get SP and EXTRA meter from the initial block.
One of the best hints I can give you is to get used to the bosses patterns and attacks, that will let you know when to block. If the attack is moderately fast enough as soon as you see it start if you block then most of the time the frames for blocking will still be in effect when the attack hits you. So its mostly a knowing when to initiate the block.
If you dodge well enough, you don't have to rely on being good at Flash Guarding. Some folks are, but rolling will serve you equally well in many cases. For example, against the cave miniboss, you can dodge its attacks reliably. For some of the Precinct boss's attacks, for example, I think you need to Guard because there's no way out. Personally, I didn't rely on it in Nightmare, but you need to do what feels comfortable.
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That's a miniboss, and a rather easy one at that. For the most part dodging is unreliable in nightmare mode.
Really there is not much in the way of tips one can give you, but it doesn't take long to get used to a boss's patterns. Just find a middle ground between the boss's attack and your guarding frames and you'll be fine.
I focused on dodging more my self i can flash guard all day but it makes playing on nightmare boardline pointless. its pretty broken while there is a penalty for it it really doesnt matter if your half decent with it. i focus on rolling and if im stuck or entirely cornered then il pull one off. and yeah some bosses do have unavoidable attacks so i wouldnt not use it at all.
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