For two mana, Luminarch Aspirant is a 1/1 Human Cleric which gives a creature you control a +1/+1 counter at the beginning of combat on each of your turns
Cards that deal non-combat damage (to any target or specifically to battles) are the only other sources that can remove defense counters on battle cards manually
Here's what design turned over: Scry Baby 4B Creature Zombie 3/2 When CARDNAME comes into play, scry 5
20 Gold Sarcophagus (SJCS-EN005): $1,699 An early staple in the early days of Yu-Gi-Oh!, Gold Sarcophagus has seen play on and off depending on the meta, and is currently limited in the game
Some of these cards will be boosted even further by the new Commander 21 cards that are incoming, but were not talking about those just yet
So take your time, do your research, and dont undersell yourself