SMITE

"As Cu Chulainn transform into Berserk mode by hitting at least 2 gods with the Spear of Mortal Pain Ability."

Spear of Mortal Pain is Cu Chulainn's ultimate ability. Has quite a large area and luckily you only have to hit two people rather than kill two with it.
Cu Chulainn has a unique ability to rage and transform into Beserk mode when his Mana (called rage, in this case) reaches 85. You gain rage from hitting and killing gods and minions. Keep it above 85 for 3 seconds and he will go Beserk.

Probably easiest in Arena again, get to level 5 to unlock your ultimate, attack minions and hit gods where you can, wait for rage to be above 85 then use your ability on two gods. This must be done before you actually go beserk.

Make sure to watch out when using your other abilities, as these will use rage.

JohnnyInterfnk has a clarification in the comments, hopefully it helps some more: "Each god you hit with Spear of Mortal Pain will add 12 Ferocity to your ferocity pool (hence the magic number of 85). This maxes at 36 (aka hitting three gods). So, if you're having problems with the early transformation (you only have three seconds to land the Ultimate when you hit 85 before you transform on your own), you can fire this ability early if you are able to hit three gods."

Thanks man!

Let me post a video since I got this achievement recently. Still don't understand how I got this because I'm sure I did the exact same steps in previous attempts. Maybe you have to hit exactly 2 people at 85? Might of hit more then that in previous attempts at this achievement.