Tirs and Fiero are Elementals, so using some Aquatics against them is a good idea. Fortunately, these Elementals like to use Cleansing Rain. That means that your Acquatic abilities will do even more damage with that cool damage buff.
To fight Tirs and Fiero, I used a Spirit Crab, Eternal Strider, and a Mirror Strider.
Against Tirs, who comes out first, I brought out my Spirit Crab first and alternated with Amplify Magic and Surge (I know my screenshot shows my Spirit Crab with Whirlpool, but you'll want to switch that up to Surge). My Surges were powerful when Cleansing Rain was up. The Spirit Crab was able to kill Tirs and then eat into Fiero's HP a good amount. Then, I brought out my two Striders and Pumped to the death.
Fairly simple fight, but I thought it might be interesting to bring out one of the Striders first and see how things might go. It was definitely harder starting with the Striders first, even though Pumping Tirs to death should have been a powerful series of attacks.
Note that I also tried the fight with a Whirlpool from the Spirit Crab rather than his Surge Ability and I don't think it went as well. The Amplify Magic + Surge + Cleansing Rain is a really potent combo against Tirs.
So, my advice (after doing this battle a few times just to practice) is to use your Spirit Crab first, then the two Striders. It was great getting to use the Spirit Crab in this fight - he doesn't often come out to play.
I actually have two (rare) Spirit Crabs, but only one is level 25. I'm eyeing my level 6 Spirit Crab and am thinking about leveling him to fight as a double Spirit Crab team against Tirs and Fiero the next time I face them for the garrison pet daily battle.
I think a double Spirit Crab team might be one of the best ways to 2-man the fight. But that's just a guess for me right now.
Also, you could probably stack some pets with Whirlpools and get some pretty good results (although, as I mentioned, I tried a Whirlpool strat with the Spirit Crab and it didn't go as well as I'd hoped).
