AD&D combat – especially initiative – is infamous for being open to numerous interpretations, and is often subject to many houserules. Demon Idol provides a defined combat procedure, and this page provides additional rules info.

• AD&D combat is meant to be abstract. This is not a simulationist game system. There will be times you may think, “That doesn't make sense!” Just roll with it.
• This is a game, not a simulation. The abstract nature of AD&D combat enables those 200 vs 200 battles of mixed troops, spells, PCs, and NPCs.
• It is only by keeping a defined combat procedure that players will be able to judge their options and make meaningful decisions. When rules are overridden by DM fiat, the game is typically weakened.

Use the alternative system provided by DMG 82. And note that a target can become unhittable under normal circumstances (21s and higher).

See Spellcasting.

To get past an area or portal being blocked off by enemy combatants, combat options are (1) overbear attack as per DMG 73, (2) defeat them in combat, (3) make them fail morale.

See Charging.