Mercernaries that can spike include:
1. Swordsmen - This merc uses a sword/shield and does powerful damage with flame sword. I once faced a build with 3 swordsmen that killed my viking in 2 turns. They are very affordable and can tank exceptionally well.
2. Spearmen - These mercs are decent stunners and can do the same or if not more normal atk dmg than a swordsmen. However, flame sword is still stronger than a spear doing normal atk. Spears in pairs are most effective because they have a skill called Lightning Spear, which decreases the AP of the mercernaries inflicted. At lvl 80, spearmen are a deadly force to be recking with.
3. Lady Knight - One of the best tankers in the game. They are considered semi-spikers because they can use Light Slash. On average I see people with lady knights do 2k with lightslash which is decent dmg. Light Slash can also combo with Vortex (oracle's skill) to do more damage.
4. Janissary - Most powerful gunner. They have a skill called Shooting Stance, which allows the janissary to do powerful normal damage. I personally like janissaries because they have a shiny aura when shooting stance is activated (aesthetics mean a lot to me lol). Mercernaries that use guns combo frequently.
5. Archer - Archers arguably do the most normal single target damage. I am a bow main but I'm going to be objective here lol. They have high evasion and are annoying to deal with since they have Silence. Silence is useful for preventing enemy mercs from using magic. There is a div 1 player called SaraPhoenix that has 3 archers in her build (archer main/archer/prophet). She sniped down an inventor in 1-2 turns lol.
6. Prophet - Good spiker with a useful skill called Concentration. Single target damage is almost equivalent to that of an archer. Prophets are commonly used in melee-oriented builds.
7. Spartan - Powerful and universally used by all top players. They do more normal damage than spearmen and have more defense since they can equip shields as well. Unfortantely Spartans are too expensive. Getting one costs around 1b in Delphi and the warcry books cost around 3-5m each for begs. Upgrading requires a lot of money also.
8. Pirate - Saving the best for last, pirates are the best spikers. With dual swords, they cause a lot of damage with Toxic Sword. If that is not good enough, they can use Blood Vengeance, which can do around 5-6k on average. Just like spartans, they are not cheap.
[I] Pros of Spike
Spike allows you to have more confidence in winning the battle because each kill gives a substantial amount of points. The number of points determine the winner in free league/titan league. You will outnumber your opponent at times. At higher levels, spiking necessary to take down a mercernary.
[I] Cons of Spike
Spiking is risky because you have to determine which mercernary to take down first. At times, the mercernary will not do combos thus causing less damage. You can not stun your opponent that well. It is difficult to stun since you mainly hit 1-2 targets.
[III] How the have the best of both worlds
Even spike builds are not completely focused on spiking. The effective method is to have a hybrid. For example spike/stun and spike/aoe.
SaraPhoenix is notorious for her spiking/stalling because she has double monk formation
She is at the right by the way.