Assist Casting (Ex): You can assist an ally with spellcasting if you have a number of ranks in the appropriate Knowledge skill equal to the level of the spell being cast (minimum 1). The table below defines the required skills for each type of ally.

The type of assistance you can render varies with your level.

Penetration: Once per round as an immediate action, you can grant an adjacent allied spellcaster a +1 bonus on caster level. Each spellcaster can only be affected by one Assist Casting (Penetration) at a time. (They do not stack)

Target: At 2nd level, you automatically aid any allied spellcasters in targeting a foe with which you are engaged in melee. Your ally does not take the standard —4 penalty for targeting that foe with a ranged attack spell, though the penalty remains in effect normally if other allies are engaged with that foe. In addition, you do not provide a cover bonus for that foe against any allied spellcasters. Furthermore, any foe you threaten takes a —1 penalty on Reflex saving throws against spells cast by your allies (but not by you).

Distraction: At 3rd level, you gain the ability to protect an allied spellcaster with whom you are flanking an opponent. As long as you maintain the flanking arrangement, your ally can cast spells without drawing attacks of opportunity from the flanked creature. Other activities that draw attacks of opportunity do so as normal.

Tactical Combat (Ex): At each level you attain in this class, you gain an ability to enhance your fighting prowess or to influence magical effects cast upon you, based on your Knowledge skills. Each time you gain this feature, choose one of the following abilities for which you meet the skill-based prerequisites. Although you can choose some of these abilities multiple times (as indicated in the descriptions), you may not select the same one twice in a row, and the minimum required skill rank increases by 2 for each subsequent selection of the same ability.

Avoid Energy: Choose an energy type from among acid, cold, electricity, and fire. You gain a +2 bonus on all saving throws made against spells that deal damage of that energy type. You can select this ability multiple times, choosing a different energy type each time. (Knowledge [the planes] 4 ranks)

Break Point: Each of your melee attacks deals an extra 2 points of damage against inanimate objects and constructs. (Knowledge [architecture and engineering] 4 ranks)

Deadly Strike: You gain a +2 bonus on attack rolls made to confirm critical hits. (Knowledge [nature] 4 ranks)

Discern Weakness: Choose one creature type from the ranger's favored enemy list (PH 47). Provided that you have at least 5 ranks in the appropriate Knowledge skill (Knowledge [arcana] for dragons, Knowledge [local] for humanoids, and so forth), you gain a +2 bonus on attack rolls against creatures of that type. You can select this ability multiple times, choosing a different creature type each time. (Knowledge [varies] 5 ranks)

Mystical Augmentation: Choose one clerical domain. The effective caster level of any spell in that domain cast upon you by someone else increases by 1, if you so choose at the time of casting. (Knowledge [religion] 4 ranks)

Physical Augmentation: Choose one of your physical ability scores (Strength, Dexterity, or Constitution). When any spell that applies a bonus or penalty to this score is cast upon you by someone else, any bonus granted increases by 2, and any penalty imposed is reduced by 2 (minimum penalty 1). You can select this ability multiple times, choosing a different ability score each time. (Knowledge [nature] 6 ranks)

Resist Ethos: You gain a +2 bonus on all saving throws against negative energy-based spells, and against spells with an alignment subtype opposed to one or both aspects of your own alignment (law/chaos or good/evil). (Knowledge [religion or the planes] 5)

Spell Disruptor: Any spellcaster flanked by you and an ally can cast spells without drawing attacks of opportunity from the flanked creature. Other activities that draw attacks of is aware of this limitation. (Knowledge [arcana] 6)