Look for synergetic traits on other purple and brown troops like “gains magic for every purple or brown” and “gains more mana from purple or brown”
I personally never made a brown or purple focussing synergy team so i dont know if theres any cards that have the leader trait. Kerberos has high costs and his main strenght is probably the double damage against burning, so if you want to put him in the first slot add a card that burns troops, or just the first troop to take advantage of that trait after you unlocked it. However there are 0 troops that have skill based burning that have any synergy with kerberos so its better to just forget about that part (i will show why later in this response)
Otherwise try to focus on filling him up quicker by the prior; get cards that increase mana gain with purples or browns and add in a purple or brown summoning card (preferably a control type but i dont think there are cards that summon purples controlledly) such as giant spider. (summons purples, but not controlledly) but a better idea would be acolyte and focus on browns instead as he already has stone link as his first trait aswell, terraxis is self sustainable (red linking) and does have the magic per brown ally so that would also have high synergy if you made a brown focus team.
Id say aziris (traited) also has good synergy with kerberos because he 1: has the magic link trait (giving more magic for purple matches) and 2: summons a free skull that kerberos can use.
But lets go with what you’re doing.
Kerberos, Terraxis. You’ll want to focus on giving terraxis those extra magics while maximizing brown gains. Add acolyte to the mix with his brown linking
Focussing on skills:
Kerb: brown purple.
Acolyte: purple blue (brown link) (brown summoning for kerb)
Terraxis: red yellow (gains no magic from himself has red link) (brown summoning for kerb)
Synergy here would be more brown or red link or burning for kerb’s skull trait.
This lacks skulls.
Focussing on atk:
Kerb: brown purple
Aziris: purple red (purple link) (skull summoning)
Spider green blue (purple link) (purple summoning for kerb)
Synergy here would be more magic for purples or even more purple link or burning for kerbs skull trait.
Cards that burn on skill:
Dimetraxia red yellow (has 0 synergy with your team) (self powering on burn though)
Salamander red green (has 0 synergy with your team) (self powering on burn though)
Chimera red green ( 0 synergy with your team) (gains 1 atk per red you dont have apart from terraxis)
Flame cannon red yellow (0 synergy with your team) (self powering on burn though)
Bombardier brown red (post patch) (0 synergy with your team) (self powering on burn though)
this shows his burning isnt the road to go unless you wnat to completely sacrifice a card slot.
Instead its better to fully focus on filling him up faster with better purple or brown links.
Putting kerberos in the second slot and looking for a card that burns people or has the leader trait and brown or purple link is the best idea. As far as that goes Sylvasi (green red) would be an even greater idea as he eliminates yellows replacing them with purples, so thats controlled power to kerberos for the second deck i made for you. He can be first position as he cant be hit by skills (trait) and steals attack making him outlast easily.
So i would advice you to make the team: Sylvasi, Kerberos, Aziris, Giant spider.
Hope this helps.