

Q1: What exactly does a "can't (perform XX on) stacked cards" effect mean?
This means that the effect prevents processing of "(perform XX on) stacked cards." For example, a "can't trash stacked cards" effect prevents trashing the top stacked cards by De-Digivolve (which trashes the top stacked cards) or similar effects, and it prevents trashing the bottom stacked cards by "trash a digivolution card" or similar effects. However, because the effect only prevents processing "(perform XX on) stacked cards," XX is still permitted to be performed on the Digimon itself, for instance by effects that tell you to "perform XX" on the Digimon.
Q2: What kind of card does "a card with X in its text" refer to?
It refers to a card that contains the specified text or icon in its name, traits, effects, inherited effects, (Rule), digivolve requirements, DNA Digivolve, DigiXros requirements, Burst Digivolve, App Fusion, Link, or Assembly requirements. For example, "a card with Knightmon in its text" would include cards with the name DarkKnightmon and cards with the text Knightmon in their effects.
Q3: Can I place BT26-085 Giant Slayer under this card when performing its Assembly?
No. The BT26-085 Giant Slayer card has no level, so it can't be referenced by this card's Assembly.
Lv.- Giant Slayer BT26-085 R NO DATA | TS <Collision> <Reboot> <Blocker> [On Play] Until your opponent's turn ends, your opponent's effects can't reduce this Digimon's DP or trash its stacked cards. [All Turns] When this Digimon would leave the battle area, by digivolving it into [Chronomon: Destroy Mode] in the hand or trash without paying the cost, it doesn't leave. --- ー
Lv.- Giant Slayer BT26-085 R - | NO DATA | TS <Collision> <Reboot> <Blocker> [On Play] Until your opponent's turn ends, your opponent's effects can't reduce this Digimon's DP or trash its stacked cards. [All Turns] When this Digimon would leave the battle area, by digivolving it into [Chronomon: Destroy Mode] in the hand or trash without paying the cost, it doesn't leave. Assembly -5: 5 different-level cards w/[Chronomon] in text or w/[Shaman] trait --- ー
- Giant Slayer BT26-085 R NO DATA | TS <Collision> (During this Digimon's attack, all of your opponent's Digimon gain <Blocker>, and the opponent blocks if able.) <Reboot> <Blocker> [On Play] Until your opponent's turn ends, your opponent's effects can't reduce this Digimon's DP or trash its stacked cards. [All Turns] When this Digimon would leave the battle area, by digivolving it into [Chronomon: Destroy Mode] in the hand or trash without paying the cost, it doesn't leave.
Lv.- 巨神兵器 BT26-085 R NODATA | TS ≪衝突≫≪再起動≫≪ブロッカー≫ 【登場時】相手のターン終了まで、このデジモンは相手の効果でDPをマイナスされず、重ねられているカードは破棄されない。 【お互いのターン】このデジモンがバトルエリアを離れるとき、このデジモンを手札/トラッシュの「クロノモン:デストロイモード」にコストを支払わず進化させることで、離れない。 ---