if you're reading this, i'm assuming you already know about Fiona's sprint smash and heavy stander cancel trick

as you may have noticed, Vindictus uses a reserve command system similar to the "reserve once" function in mabinogi

if you left click to attack in the middle of a previous attack, it will automatically attack a second time as soon as possible

for consecutive sprint smashes with the heavy stander cancel, there are two points where you can work on to speed up the whole thing:
how fast you transition to heavy stander, and how fast you transition into the next sprint smash

in terms of mouse mode controls, I personally do this:

hold the direction you wish to sprint smash
hold LShift
right click

BEFORE SPRINT SMASH COMPLETES
{
release the direction you sprint smashed in
release LShift
hold spacebar
}

react or time to Fiona's guard motion with

tap/hold W //if you are sprint smashing forward, hold it; otherwise just tap it
release spacebar

then start from the top again, do note that the first three actions at the top must be completed before Fiona finishes her heavy stander motion; otherwise the next sprint smash will not come out immediately

note: if your character ends up doing a sword#0 or hammer#0 smash, then you've released the directional key and shift too soon