Askr is an amazing unit when supported by L!Robin, Y!Eliwood and Gotoh.
His build (globally base kit) :
Spe) Bonfire / Sacred Cowl
A) Distant Counter / Atk Def Ideal
B) Atk Def Bullwark 3
C) Opened Domain
S) Bonus Doubler
Bosses tanked :
1- Hector
2- Duma
3- Dagr
4- Roy
5- Leif
6- Edelgard
7- Corrin
8- Micaiah
9- Caeda
10- Nanna
11- Myrrh
12- Robin
13- Hinoka