I have some personal issues when I face Compound Beams. I am not sure why Compound Beams activate a Stasis effect which applies instantaneously. I can't do anything better but fire at my weapon cuz there's literally no way of escape. Tried dashing away but the enemy simply attacks with the EMP. Some players also knew how to "permanently" stun their opponents until they are killed.

There is no 'clear' animation which indicates it has fully charged. I use a Vertical Target Assist and in the crosshair, purple energy seems to fill until the entire white part fully becomes purple (I also use my ammo indicators which will help me reassure how much percentage it has got charged) then we'll have to release the weapon and tap to apply an EMP which is of 30m range and can stun multiple opponents. The blast also deals very high damage, and easily one-shot rare mechs. Compound Beams function exactly like Particle Beams but you'll have a shorter range for both the beam and EMP blast.

For near-infinite stun effect, You'll have to recharge both weapons, then tap fire button of one weapon immediately to affect opponent with 1.5s EMP (make sure opponent stays within 30m range or you'll lose most of the damage and the EMP) then use the remaining blast to stun the opponent again while recharging the "emptied" compound beam (sorry if my instructions aren't clear but I hope you'll understand). By repeating these steps you can stun for a lot of time 

I find Citadel + Compound Beams very hard to counter (because of too much movement speed reduction from both Stasis and Citadel's ability) if I'm not using either a Guardian or Outlaw or a Tailsting. Someone help me by giving advice on how to counter Citadel+Compound Beam combo 

for that i would advise to use exactly what you already use :) guardian or outlaw. And stay out of range, with Citadel staying out of his ability range applies to any weapon he is using.. yiu can also use compound beam against likes of him and wortex. Compound beam seems to immobilze wortex if he is using ability :) 

sometimes it feels like slow effect does not apply immediately but it could be because it fires against faster mechs.. idk

There are many mechs and weapons combos difficult to counter.. and a lot depends on your luck of matchup, not your skill.. For example if you have outlaw with gemini and some sort of wortex with revoker you would potentially dominate if round matched outlaw against deathwalker and wortex vs eclipse, but likely fail miserably if it was the other way around, outlaw vs eclipse and wortex against deathwalker.. and you cant much predict how it will fall.. you may be a sniper against seeker and not much you can do about it.. ideally you would need all your HS mechs to be as much universal as possible.. 

Try to stay away from Citadels field in the small HS maps? 

Maybe the field is a bit to big.

I think another way to fight it would be using deathwalker if you activate the ability before you're in the seismic field

I think compound beam in beam should change colors based on how much the pulse mode is charged so there's a visual indicator.