Is that normal when Gefjon take 11xx damage on Magni

No, it was begining of match, that is first Skill of Gefjon and my Magni just got few normal hit so he die with one-hit

Adding image of Gefjon team

And my team also

You really should try to play with color stacking. You attacked a yellow tank and therefore should try to take 3 purples. Even a mono purple team could work very good.
No time or place to start a big discussion here, just simply try it, it definetly needs time to learn.
Strong rainbow teams only work if they use the top heroes (f.e. alberich, seshat, gravemaker, ariel, whiterabbit)

My maxed magni has 1300 life, plus troops, so i dont think your magni was oneshotted by gefjon.
More likely he has been hit by 2 or 3 slash hits before gefjon fired. Proteus and skadi have high attack and easily hit glass chin magni with 200 to 300 slash hit damage

Gefjon is a monster. Faced her in raids and she almost killed alfrike with one hit, gefjon however got poisoned and when alfrike went off, she killed gefjon and crippled the other team. Gefjon impressed tho, one hit and almost killed alfrike. My alf only has one emblem or maybe 2 tho

I don’t have enough hero so, sometime if I don’t have enough hero for some color, I will fill with rainbow team