how does one use silver keys in doom tower

You have to reach a "secret room", or rebattle a boss after you've defeated it the first time with a gold key

You've finished level 22, move on to level 23, don't worry about making 22 better.  If you're farming, farm 20 or 25.  Same team should work for 23 (Void) but Coldheart needs WAY more damage over 9 minutes (70% Crit Rate, don't go over 75%, ABSOLUTE MAX Crit DMG. (don't worry about spd).  You don't have a 60% DEC DEF champ.  Maybe use FATMAN instead of Duchess, Visix instead of Alure (only use her A1 in the Boss), if you have another Coldheart, I'd build her.  I use Visix (A2 in round 1, A3 in round2, A1 in Boss), 2 Coldheart (A3 , Mithrala (Waves/60% INC DEF) and Lydia (60% DEC DEF v. Boss). There are others for 60% DEC DEF, Deacon, Zargala, Uugo, Ninja (triple hit A3, 60% DEC DEF A1), Fatman (Ally ATK and 60% DEC DEF) may work if you can get his timing right.  You don't need AOE DEC DEF, single target (obv) will work.  I don't have Staltus, so I'm not sure how good he is PVE.

Edit: Ninja's triple hit is A2, not A3, my bad

Big key is to get his shield down, then hit him with the 60% DEC DEF, then smack him with Coldheart A3. use Coldheart and visix to keep his turn meter down and keep walloping him, should noly take 2-3 minutes per level.

CHs A3 is capped past stage 20 so she's tops for getting the shield down but only so-so for the damage.  Fahrakin would definitely be a big improvement over Duchess for sure.