One thing that I have learned prior to soloing L4 missions for surviving Security missions (i.e. pve combat) is that how you decide to tank the missions in the first place. When I started running security missions I decided that I would run an armor tank using an active tank at first and then a more veteran player suggested that I switch to a passive tank so that when I was capacitor neutralized, I wont lose my ship right away. To support this though I needed to train my armour related skills to a level (3 or 4) high enough that I could then concentrate on my other skills while running missions. And as a result as I survived the missions and completed them my faction standing increased to the point now that if I so choose I could transit to low sec and run L5 missions, but I decided that staying in High Sec was more profitable by salvaging (and looting) L4 Security missions and including bounties and rewards.
IF you also train the relevant compensation skills to max level, you will achieve a maximum of 25% increase to the associated damage type. An example would be a base passive armour tank using a megathron… the hull starts with 10% protection against explosive armor damage and you add a basic energized explosive membrane which provides a -30% bonus for explosive protection the resistance for Explosive Damage will jump to 37.0% with an untrained exp comp skill (lvl 0). At Level 1 for the Explosive Armor Compensation Skill you get a 5% increase to the Explosive resist and for every additional level of the skill that you train you get an additional 5% per level up to level 5 for a total of 25% additional protection.
In some cases you can add an armor pump to the rigs that you use to adjust for any holes that the ship modules that you are using to offset the different damage type combinations, but if you are using an omni-tank (i.e. all damage) use an anti-explosive pump and energized armor nano membranes to achieve a 75% damage resistance on each damage type (EM, Kinetic, Thermal, and Explosive).
For a Shield Tank, I believe that the same principal will apply with respects to Damage Type Resistance(s). For this can a shield tanker verify this for me.
By the way, depending on the type of the ship you decide to use, pay attention to the description and traits of the ship will also provide a bonus in the protection of that ship as well