There are usually 6 types of champions you will use: Straight Damage, Bombers, Support, Debuffers, Speed Boosters, and Enemy Max HP.
Trunda
Lord Shazar
Duchess
Madam Serris
Arbiter
Septimus
When gearing champions, there are several things you need to consider:
Does this champion need ACCURACY? If they do, it is important to have enough accuracy stats to debuff your targets. A general rule of thumb for Dungeons is acc = dungeon stage x10, and caps out at around 225 accuracy for Stage 20 to 25. Or, you could go to Hellhades' Raid Stages tool (more on that later), create or login with a free account, and find the "Encounter Type" you are looking for. The yellow number next to the BOSS' resistance is the accuracy you want. For Arena, you do also want 200+ accuracy and possibly going up to the 800's or 900's (you could also use an increase acc buffer like Yoshi, Lady Kimi, or Klodd Beastfeader).
Do I want to build this champion for a little DAMAGE? If so, what is the stat that they do damage off of? HP, Atk, Def, or Enemy Max HP? Take note that some champions will do damage based off of more than one stat.
Can you get enough CRITICAL RATE on damage dealers? If you can get enough crit rate (100% or less depending on extra crit rate from skills) then you do have the option to use crit damage gloves to gain more damage.
Can you get enough SPEED? If you can get enough speed, you have the option of using Atk%, HP%, Def% boots instead of speed boots to gain, once again, more damage.
Straight DMG:
For damage dealers, you generally don't want to have much resistance or accuracy as not to hinder the damage output. However, some sets people usually use are Cruel, Savage/Lethal, Stoneskin, or Immunity/Untouchable. For substats, you want Crit Rate, Crit Damage, Speed, and damage stat.
Gloves: Crate/Cdmg
Chest: Damage stat
Boots: Speed/damage stat
Ring: Damage stat
Amulet: Cdmg
Banner: Damage stat
Bombers:
For bombers, you want high atk since that is what bomb damage is based on as well as accuracy to land the bombs. You usually don't want to build too much damage on it (no more than 25% enemy HP), due to the extremely broken Resurgent mastery. For bombers, you want atk, speed, and acc substats.
Gloves: Atk%
Chest: Atk%
Boots: Speed/Atk%
Ring: Atk
Amulet: Atk
Banner: Acc
Support:
For support champions, you want to have high HP and defense stats and lots of speed in order to keep your team alive. For substats, you want HP%, Def%, speed, and maybe acc.
Gloves: HP%/Def%
Chest: HP%/Def%
Boots: Speed, HP%/Def%
Ring: HP/Def
Amulet: HP/Def
Banner: Acc/HP/Def
Debuffers:
For debuffers, it is usually a crowd control or decrease atk/def champion. You generally want decrease defense champion to go before your DPS champion. For substats you want speed and accuracy.
Gloves: HP%/Def%
Chest: HP%/Def%
Boots: Speed/HP%/Def%
Ring: HP/Def
Amulet: HP/Def/Atk (Just need acc substat)
Banner: Acc (Hopefully with speed substat)
Speed Boosters:
For speed boosters, all you care about is speed. In Arena, people usually aim for 300+ speed. On speed boosters you don't care about what the mainstat is, you just care about having more speed. For substats, you want speed and accuracy if u need it.
Gloves: Any (Just need speed substat)
Chest: Any (Just need speed substat)
Boots: Speed
Ring: Any
Amulet: Any
Banner: Any/Acc (Just need speed substat)
Enemy Max HP:
For Enemy Max HP champions, their damage stat (atk/hp/def) doesn't really matter. What matters most is crate and cdmg. For substats, you want Acc if needed, speed, crate, cdmg.
Gloves: Crate/cdmg
Chest: Atk%/HP%/Def%
Boots: Speed/Atk%/HP%/Def5
Ring: Atk/Def/HP
Amulet: Cdmg
Banner: Acc/Atk/Def/HP