This server was RagnaXP which provided unranked slim white potions.
However you are missing the point a little bit: the knight does not need to pot here because he is not taking damage. The reasons for this are:
full demi human and neutral reduction (40% + 20%)
max vitality build (~120 vitality is possible without food)
peco peco armor (notice that the knight is freezing): +10% max hp
kyrie eleison skill description: This effect will block MaxHPofTarget*(10+2*SkillLV)% damage OR (5+SkillLV/2) hits, whichever is reached first.
with apple of idun (remember that your server is running 7x7 song aoe) it is possible to obtain ~30k max hit points
This gives priests a <1 sec cast time skill with no cooldown that gives knights 10,000 extra hp which means a monk needs to do ~+20k base damage on asura (thanks to 40% 20% reductions) just to pierce KE shield. With a bad monk build (high dex), it's easy to do less than 10k damage, especially if the knight is standing in magic or arrow shower so that lex aeterna is not guaranteed.
Additionally, its much easier to spam KE on one target than it is to summon 5 spheres, fury, summon 5 more spheres, and then asura.
A number of other teamwork focused ways exist to counter asura strike:
Curse bow/arrows means that monks need to sacrifice their weight limit to carry items that clear curse, otherwise they cannot walk out of their guild to asura
Quagmire makes asura cast extremely slow
Spell breaker interrupts asura
Dispell clears devotion which means magic or arrow shower will make asura impossible to cast
Safety wall blocks all asura damage
Lastly, knights can also hide asura strike although this can be pretty difficult.