The last new weapons for aliens : a "fusion rifle" with the new damage type which is really armor piercing (old acid) and able to one shot a HWP but usually don't kill a soldier in less than two shot.
The eleriums shell are xcom version of this damages types and can made ammo for sniper rifle, autocannon and shotgun (don't know yet if the shotgun's are useful)
I have also finally choose to totally change the armor of mutons : decrease armor to 8-10, and up theur damages resistances to 60-40%. I have found tiers 3 aliens too many armored and make pistols and many rifles nearly useless. With this modifications, mutons are long to kill, but can be injured even by pistols, while gazer and ethereal are nearly immune to small arms, but can be easily one shot by anti-tank stuffs. (mutons can't : because they haven't really armor now, anti-armor weapons treat them as unarmoured target, and because they have plenty HP, they can survive many shot of for example sniper anti-tank shot, or fusion rifle shot).