dont boost your def.... only boost your reduct systems as rayray said.. me personally i have 5armors/3shields/4cloacks. i use different card set on different situations. ill share you some of my card combos.
on armor:
gr+tao vs bio and champ
gr+rsx vs bio "if you have no fcp"
angeling+tao vs lk,GS,sniper,prof,wiz and other who uses elemental damage..."except shadow"
puppetring+tao vs lk,GS,sniper,clown,stalker and all who uses ship capt hat
orc lord+tao vs sinx and to take down champs with you... XD
on cloacks:
2noxious vs all range damage especially sniper
2skolls vs all thana IP users
2raydrics vs champs and bio
dev+ray vs lex+asura champs
on shields:
usako vs general damage
GTB vs AoE magic especially on wiz
maya vs single target magic prof and others...
you can make a combo card on your own. depends on your fighting style and situation that you are in to...